holy shit long post... sorry, i was just typing away!
this is a good question.
learning about gameplay - not sure if this is exactly what you're looking for, but i put most of my thoughts about this in two gameplay articles. the first one is
here. it won't tell you exactly how to design a layout, but it will make you better at picking out flaws in your design during the drawing phase, which will save you a lot of time in the long run.
since i've written these articles, i rarely find huge gameplay flaws in my layouts that force me to go back to the drawing board after the planning phase.
how did i gain the knowledge to write these articles? well, i practiced a lot, but that's not the main thing. the most important thing is i studied the classic maps and tried to figure out what makes them work. this is the best practice you can get, in my opinion. whenever i decide to start mapping for a game that isn't quake, i'll probably spend most of the first month doing this instead of making a bunch of crappy half maps.
in terms of my exact process, what i do nowadays is this:
drawing - this usually starts with an idea in my head for a room that would look really cool and have nice gameplay. i can't give you any advice for how to get ideas like this. not to sound too cliche, but these usually just come to me in a *flash*, and i sketch the room out in a top-down view on scrap paper. usually i make this room in the editor right away to see if it's actually worth turning into a map. the MH rooms in kara and hydra happened exactly like this. the initial room always holds an MH or an RA, btw, since i start with the most important areas of the map first.
blockout - i use a few stock gothic textures and that's it. a couple different colored floors, one wall texture, a stair texture, a decent skybox, and any special textures i need like grates or bouncers. i don't go off the 64 grid unless i absolutely have to. i compile the map a lot as i block out to test scale and jump distances.
repeat - once i have my first room figured out, i move onto the next most important room, which holds the RA or MH. again, i start by drawing it top-down and i usually get ideas based on two things: 1.) first, i have a list of ideas on how to create "traps" around power items for interesting gameplay. i might share this list in another post eventually. 2.) i try to expand on whatever themes came out of the first room i designed. for example, in kara, i noticed the MH room had a lot of small (32-128u) elevation changes which created cool jumps, so i decided that would be the theme for the whole map's gameplay. that gave me the idea for a
central pillar in the RA room. when i really can't think of stuff, i get ideas from other maps. for example, i probably wouldn't have thought of that central pillar idea if i hadn't seen something similar in flipout's map,
achromatic.
once i have my two main rooms figured out, i work on the connections between them. the connections between RA and MH are the most important because they will determine how players run the items on your map. i basically rotate the rooms around and figure out what the best positions are relative to each other for cool hallways.
after that, completing the map is usually pretty straight-forward. i use the entrances/exits of my existing rooms to think of ideas for any side rooms/hallways that could connect them. once that is done, i jump around in the map for a while and mess with room/hall sizes and shapes.
entities - my item placement strategy can be found in this
article. once i have all my items placed, i try to do a pretty good job of lighting the map because i don't want to make my alpha testers go blind :0.
test - get as much feedback as you can on forums while you play the map with friends or bots. don't start detailing the map unless you're pretty sure no more big structural changes are going to be made.
detail - as far as my process for detailing the map, that is something i have been developing more recently. my project hydra was an epic fail in this regard. i dove into the detailing process without any concept art or practical theme in mind. i started detailing the side rooms first (big mistake). i had to change a ton of stuff and the detailing has taken twice as long because of it. one thing i did that was smart was i detailed and lit one area to completion and released screenshots before touching any other parts of the map. this allowed other people to give me feedback about my theme and put me on the right track before i wasted too much time on the rest of the map.
for kara, i'm doing things differently. i started by nailing down my theme exactly and collecting tons of concept art with ideas for the walls, floors, ceilings, props - basically everything i could ever need. then i figured out what materials i was going to use by collecting some placeholder textures. i started by detailing the bare-bones structure of my map. since my map has a realistic theme, i needed to figure out where windows and wall supports would go from the beginning. i also thought about how my ceilings were going to look. i did this in each major room until i had a solid first draft of the whole map. only now am i moving onto the finer details, like the actual window designs and other things. soon i will finalize textures, and then move onto the smaller details like props (lamps, machines) and environmental stuff (cracked floors, leaves, vines).
here's where i am at right now. basically the point of all this is to work from big ideas to small ones so the whole process is more efficient.
