Where can I find good resources for making maps in SC2?
Are there any good tutorials out there on the map editing tools, as I always find the experience of making custom content
What goes into the design of a good map?
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There are very few "good resources" on using the Galaxy Editor, and I have yet to come across a simple and straight forward hands on tutorial for it. A few sites have been helpful in the past to me:
SC2 Mapster has a few bits of documentation, as well as a reference on built-in functions of the Galaxy language. I found this rather useful.
IronManSC has been doing casts on the topic, and I've found them very helpful as well, although they are not "beginner level".
You should also look at The Hive Workshop, a Warcraft 3 modding community. Many of their members also do SC2 mods, so you will find a lot of useful information on the forums. Just google for them, SXC won't let me post more than 2 links for being a newbie.
There is no official documentation, as far as I know, and Blizzard doesn't officially support the Galaxy Editor. Maybe things will change when Heart of the Swarm comes out, as it carries a lot of updates to the editor, and even some new tools.