After having looked for an answer for quite some time now, I’ve decided that just straight out asking for one on this forum might be faster.
All of your mapping bibles and advice is very nice, but I have no idea how to apply it, since I don’t know which program to use.
Basically, I’m interested in constructing level geometry and entities, setting up a map. I’m not interested in making textures, sounds and such - I hope that I can get all of this from various source sites.
I realize that this may not be the smartest first post to make, but I hope you’ll bear with me.

