hmmm, many of these steps get omitted depending on the map, but:
brainstorm - usually a theme or gametype will hit me and then i have to think of how to build a map around it.
research - very rarely, for a specific theme i'll go do some research and collect pictoral references to work from.
layout sketching - also very rarely, i'll sketch out the map progression before i open hammer.
open hammer - first things i do in every map is create one floor brush in the approximate size of the entire map, then in the center i place 2 props, a CP and a Heavy, for scaling references.
place some blocks - to get an idea of how the map is going to be laid out, I place giant block brushes in all the areas where there will be buildings in the approximate size they will be.
start cutting shit up - next i'll start making this blocks look more like buildings, give them floors, rooms, windows, doors.... all very roughly though.
color code - next, i'll go over my map and apply solid color textures to everything according to what that object is. this not only helps me identify objects which will have similar textures later, but for playtesting these base colors are enough to play with and see lightmaps with.
place the mechanics - from here, i'll start placing all the entities that will make the map work, like spawn rooms, flags, and CP's.
compile and test - at this point the map is complete enough that i should be able to play a round on it and win. i'll keep tweaking and testing until this works exactly the way i want it.
makin it look pretty - now i'll go back and do all my custom brushwork and lay out some props to flesh out the till now very empty map. i'll also usually add all the lights during this step aswell.
playtest again - now i have to go back and see if what I've just done is causing my map to lag a lot or if it impairs movement, allows for exploits, etc... usually this portion of the test is where i want to play with others and get feedback.
make it look prettier - so by now, people are getting settled in with the layout of the map and the shit in it, most of the brushwork is finalized, and so now it's time to go back and texture everything. this step i hate the most, because texturing frustrates me to no end. i'll also go back and tweak all the lights in case there were things I didn't like about them earlier.
add health/ammo - now i'll go and place health and ammo packs around the map, because, after the next compile, people are gonna be playing the "release candidate" of the map, any changes i make from here on are probably gonna be minor.
test! test! test! - get the map out there and have as many people play it and give feedback on it as possible. hopefully there won't be very many major changes i'll need to make, and preferably i'd rather not make any at all, lol...
So far, I haven't completely finished a map... I'm like that with a lot of things. Mostly because the further along in the process I get, the more bored I get with working on the map.