First of all let me make it clear that a map maker needs to be curious, creative, like to play with details and manage to find out of things him\herself without asking people for help unless atleast half an hour of testing is done without success. If you ask people for help once you get an idea, without even trying yourself to find out how to do it, just because you've never done it before, then you'll never became a good map maker!
Also do note that making a really good, popular map takes months to complete! And Im talking about working several hours with the map several days each week! If you dont think you're that patient, start with a small idea, a small map, and do not hope for a 4+/5 grade. In general, dont hope for a very good grade on your first maps. Most people have to play around with the editor for a while before they manage to make a really good map.
If you fill these requirements, let's move on to the real stuff:

1. Originality
The most important thing in map making is to have an original idea.
You can make a good map and get some fans by using an old idea if you do it really good, but your map will never be one of the maps "everybody" is playing all the time and your name will not be very well known.
When creating a product for thousands of potential users you must have something to give that no other products of the same type has.
Take your favorite wc3 map as example. What makes your favorite so special? If you think about it you will realize it's because the map has something to offer that no other maps has. Often this thing is the gameplay or some functions that are unique for that map.
So if you want to make a really good map, that you want to be among the maps "everybody" is playing, you need to have something special and unique to offer, like a brand new idea of gameplay or new useful game functions (then I mean something big that affects the whole gameplay).
Things like new units, sell functions, leaderboards, new heroes etc alone is by far not enough to make the game popular.
Take Massacre TD as an example. What made this game special is the amount of creeps. Most TD's have like 10-30 creeps coming from 1-4 spawning areas, while the first versions of Massacre TD had 10 spawn spots spawning 420-840 creeps depending on the difficulty. The 2 latest versions had "only" 290 creeps from all 10 spawn spots. "I love killing lots of creeps!" That's the main reason why Massacre is any popular

Another word for map idea can be concept. Examples of used map concepts are towers figthing creeps, heroes fighting creeps, heroes fighting heroes, creeps fighting creeps, towers fighting towers, tag, heroes doing summoning, playing survivor in the woods, maps made from movie stories, surviving oncoming waves of enemies as long as possible, hero figths with user selectable skills, heroes morphing into new heroes, etc.
Tip: I dont know any maps with playerowned creeps fighting npc towers or playerowned creeps figthing npc heroes.
So if you have an idea for a map, that is really original, no other maps use the concept and the idea is something big that affects the whole gameplay, then you have a very good start!
In the start while being a noob map maker I really thought it was hard to come up with something original. But after I got alittle more experienced and I knew what is possible to do, then I got a flow of ideas so big I can't manage to make all the maps fast enough!
It all comes with experience

2. Learning the editor
It takes some time to learn the editor by heart, and the only way to really learn it is by experience. You can do everything with the wc3 map editor, the only thing that prevents you from doing just that is time and experience. The importance of terrain and layout varies alot from gametype to gametype, and the sounds editor is often very little used, but the unit editor and trigger editor usually stands for about 90% of the work together. You already know by playing experience how important the units are, but the triggers are just as important as all the units together. Although a noob map maker might have no idea at first how to use either of these editors, they are really easy to learn. The triggers are all logical and the units editor is just a matter of remembering everything to change when you want a new custom unit. There are many details to set when you make a new unit, and it's very easy to forget some of them, ending up with a unit that doesn't work like it should.
Another part of the editor that can be really helpful is "Gameplay constants" and "Game interface" on the "Advanced" menu. There you can change how many levels a hero can level up, how much a unit will be slowed from the slow ability, what the food limit should be, names of weapon types, how much damage different kinds of weapons do to different kinds of armours, change images of all buttons in the game +++.
When you wonder about something in the editor you should try for yourself for atleast 30 minutes before you post a question at a forum. When you think about posting a question, do note that it might time several hours, maybe a day or two before you get an answer. And if you post a question that has been answered tons of times before, is already answered or commented in the FAQ or is as easy as cake to find out, be prepared that you might get some annoyed people at your neck. You'll understand when you become one of the "big guys" who have to answer the same questions over and over and over and over again. So use your brain before you start yelling for help. You know, by checking the FAQ and reading old threads about your problem you might solve it much, much faster than you would by posting a new thread!

3. Game functions
Game functions is a part of the map that is often not very neccasary but will improve the map's quality. Examples of game functions are selling functions, "give gold to player" functions, repick of heroes in hero maps, voting for difficulty, gold gambling functions etc. Good game functions can be like the cream on a cake, it makes the whole thing more enjoyable.
They add more work, but they will all make the map easier to play, remove possible annoyance from players and in general make the players like the map better. New, original game functions to an old map idea will attract more players to the game with less work than implementing a brand new map idea.

4. Perspective
A useful thing when you want to make your map popular is to pretend you're new to the game and think about everything you would like, dislike and new things you would wish to be implemented. This is more helpful than you might think, because the common player will have a pretty different view at your map than you do. Think of everything you think would be exciting and fun, think of everything you think would be discouraging with the map, then fix and improve! You will sooner or later come upon the problem "should I put in alot of extra work to make the players a bit more happy, or should I drop it, save time and get a slight lesser grade on the map?". Let me tell you: It will be worth it! The more work you put in your map, the more game functions, the more options, the better balancing, it will all increase your maps grade and increase the chance for your map to become popular! 1 thing alone probably wont do that much, but you will stumble across enough of such things to really raise the map's grade.

5. Polishing
If you have come up with a really good and original idea for a map, you have already secured your 4+/5 rating on 1 condition: That you really take your time polishing the map. I do TONS of testing on my maps before I even start thinking of a beta version, and I do TONS of testing before I even start thinking of an official release. I do not want people to complain about things like "wrong info on that unit", "bug in leaderboard", "bad placement of that building" etc. When you do the testing, think about everything you think people might complain about and point at, anything that could ruin your 4+ rating!
The easiest way to remember every small detail you need to fix is to point at the thing with your cursor ingame, write a message (without posting it, so noone else has to see it) and then take a screenshot. Then when you're done with that testgame you can watch all the screenshots 1 at a time, fix the details and delete the screenshots when you've fixed the error\whatever on the screenshot. You should also use this if you get an idea for the map while playing. I usually end up with about 3-10 such screenshots from every game of testing I do.

And for the triggers: DO take the lang way, dont do all the shortcuts! Taking all the shortcuts will often slap you in the face later, when you realize you have to expand the map and the shortcuts give you even more work in the long run. You should set up the triggers in a way that makes it easy for you to change them later. For instance, dont use the "if / then / else" function, use the "if / then / else (multiple functions)". It turns out pretty often that you need a multiple IF sentence instead of a regular one, and it's also much easier to find bugs and problems in a multiple IF sentence than a long regular one that often is too long to be fully viewed. (IF sentences is a common place to find bugs).
Another important thing is that you should always do testing of a map version before you release it, because sometimes when you fix bugs and problems you cause another problem/s to occour, and you wont know if you dont test the map.

6. Map size
By sacrificing the size you may add custom models, custom sounds, mp3's, introduction pictures etc to the game. But the map file will easily become very big, and no one likes big map files that takes ages to download at the battle.net, which I belive stands for about 95% of the rate at which maps spreads and becomes popular and known. Big maps will make the players unpatient and leave the game without having downloaded the whole file, more players will get kicked because the host is not patient enough to wait for them, and the rate at which the maps spreads will be decreased considerably! This is a problem every map maker has to decide on his own, personally I avoid using any files not already included in the map editor.

7. Protection
There's alot of discussion among map makers if it's a good or bad thing to protect maps. Im a supporter of map protection so I will tell you how to do it. But first I will tell you about all the good and bad things about protecting your map, to let you judge yourself.
+Without a protection, anyone can change a map against the authors permission. The author has absolutely no control of the map.
+Without a protection, anyone can steal the map, by releasing it in his name and thus stealing all the credits.
+Without a protection, anyone can copy things straight from your maps, putting them in their maps and claim it to be all their ideas and made from scratch.
-Without a protection, everyone else can learn from your map, improve their map making skills and make new, good maps based on yours. (did you know Massacre TD was built on Taishos TD?)
-A small mistake might cost you TONS of work.

Well here's how you protect your map in a 99% secure way, not even the blizzard team will be able to crack it!
All you need is the wc3 editor and an empty word file. Put the word file for instance on your desktop, just make sure you know where it is.
The reason why it has to be empty is that the file itself will not be used, and a file that is not empty will add extra "weight" on your map. Big maps is not a good thing, so use an empty file. (you can also use for instance paint files, just make sure it's as small as possible)
In the editor, editing your map, press f12. You will then enter the Import Manager. Right click in the window, choose "import file" and choose the empty word file. Then doubleclick the file you imported and "Imported File properties" pops up. Click the box for "use custom path", type in "war3map.wtg" in the path and click ok. Now comes the really important thing: Save the map in a NEW file! You wont be able to open this map after you've closed it!
If you save the map on top of your old map file and close the map editor, then you have my permission to go knock your head in a concrete wall. Been there done that!
When you edit your map, always save your map in 1 protected file and 1 unprotected file, use different file names and put them in different folders to tell them apart. Now always make sure you play the right map file on the battle.net, pick the right folder with the right filename so that you dont give away the unprotected one.
* I've just found out there really is a way to unprotect a map, nomatter how you protect it. However, very few people know how to unprotect maps, and this way of protecting it might be the most secure. Please do not ask me how to unprotect a map, I will not help anyone unprotecting other people's maps. Personally I wish it was literally impossible to unprotect any protected map.

8. Spreading the word
When your map is finished and ready to be released, you want to find a good way to make it well known fast. The most important way of releasing it is to host it at battle.net. Keep this in mind when you create the game, because the more playerslots you have in your game, the faster your map will spread.
The absolutely best way to do it is to play it alot with all slots used. Some players can't host the game and are not able to host for you later, but they will populate the games other players create. You know a game with only 2-3 players is not as fun as a full game, so the more players know your map, the better, because it makes sure new players dont leave because of almost-empty games (this is just a minor grade of affection on the spreading, but still). Also, I use to create host games called "no dl, host away", where I have 1 slot open and let people download the map. Then I go to sleep, school, work, watch tv or anything that can be done while wc3 is running. You should only have 1 slot open because then the game will automatically be refreshed every time a player leaves the game.
Keep hosting it for as long as you have patience to.
Btw: If you get alot of "OPEN THE SLOTS YOU B*D F*K YOU G*Y *SS N****R" that's just normal... You can do like me and msg them with "watch your mouth or I'll ban ya from my map!"(even if you're not going to) Some dont care, but some get scared;)
At the same time, release your map at http://maps.worldofwar.net/ so that even more players can download it here and give your map a grade at the same time. Barely 1\10 of all the people who download the map will give you a grade, but it's fun and very encouraging when you get a good grade. Last, upload your map at www.wc3sear.ch. That website is not as good as the worldofwar site, but your map will get uploaded right away so people dont have to wait to download it like they have to at worldofwar.net.

 

If you follow these steps carefully you may become a very good map maker! And we need more good map makers, so get going!;)

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