Each persons methods are unique to them, but the thing that is consistent in all methods is inspiration. You can't make a (good) map without being slapped in the face with a totally awesome idea. The idea can be anything from wanting to use a specific prop (not a good route to start a map imo) or having a new idea that you think could alleviate some of the flaws on a game-mode to just liking a shape of a building you saw once, or wanting to make a twisty cave. From there you can make your map, whether that means drawing it out in great detail or going in all guns blazing blocking it out in hammer.
Personally, I visualise a feature of a room or and entire map, create it, then let the map grow around it. Not a good method, I wouldn't suggest it to anyone...