Making maps may take a lot of practice and time to perfect, but the actual process of creating one is much easier than mapping for other games such as TF2.
THE BASICS:The game requires two .png images, one for the collision map, and one for the actual map art.


- The collision map, also called the wallmask, shows where players can collide with the environment. Solid areas should be colored black (RGB 0,0,0), and empty space in white (RGB 255,255,255). THE BOTTOM LEFT PIXEL OF THE WALLMASK NEEDS TO BE WHITE!
- The other image is what is shown ingame, and is called the background. The background should represent graphically what areas are solid or not; the wallmask is not actually displayed!
Garrison Builder lets you convert these two images into a working map. Whatever part of the map (WM or BG) you load second becomes the compiled map.
Other tips and trivia:
-Maps are zoomed in 600% size in game.
-Players are approximately 6 pixels high in game.
-Players can reliably jump up to a platform that is 9 pixels high, Scouts can double jump about 16, and the rocket jumpers can make about 30 pixels.
-Ramps are seamless to run up when they're 1 pixel rise/1 run slope. You can't run up a 2 pixel high block.
(original text by fishmatt, updated by Agh and Chu)
TUTORIALS:basic:
Comprehensive guide to map making (also a Haxxy 2012 entry)Garrison Builder TutorialTater's GB tutorial (incomplete)Video Tutorial for making CTF mapsReference sheet for Garrison Builder by tiscooleradvanced:
How to make your map not suck - List of the most common mistakes, which make the map unbalanced.
Tips on drawing maps - A focus on the art side of mapping
Checklist for Map Makers - Very useful if you run into technical problems.
Agh's mapping lessons - I talk about everything here.
The next generation of mapping (Variations!) - Ozoh is showing us different creative ways to create new game modes.
OTHER STUFF, THAT SHOULD BE USEFUL TO YOU:How to make a locator file locators are obsolete since update 2.3, ignore any reference to them in any guide.
How to host a game with custom maps