How To Develop A Map That Works
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This page is intended to have advice to the fledgling mapper about how to start a map so that it's got a better chance of being playable in the end.
The golden rule for this page is: it's much, much harder to change the fundamental layout of a map once you start adding detail.
Always start with pen and paper. Draw up your basic idea. Then think of as many variations on that. Think about where the meeting points will be (when the two teams see each other for the first time). Think about snipers and campers. Bombsites, their accessibility and defensibility (both for CTs and Ts). That sort of thing.
When you finally do fire up Hammer, create the basic map structure using dev textures (type "dev" in the filter box) and big, simple blocks. Place some spawn points, some buyzones and some bombsites. Don't bother with lighting, props or fancy textures. No stairs, just ramps.
Fire the map up and see how it flows. Make sure meeting points are timed correctly. Make sure visibility works the way you intended. Make sure spaces feel the way you intended.
Then unleash 20 bots on the map, and let them go at it for an hour. If the result of that comes out significantly in favour of one side or another, try to figure how to re-balance the map. Watch where the bots meet up, or see whether there's a camping or sniping position that you didn't think of that gives one team too much of an advantage.
Once you're sure that the map works correctly, then you can start thinking about making it pretty by adding textures, details, props, sound, etc.