I'll be honest, the single best way of doing it is to use Kara's tutorial, it's pretty much a collection of the tutorials we all learnt on, so we can promise they work.
I'd suggest, first of all, use the links in the tutorials for the modding stuff, and start learning about the file structure, the .vp files and the .tbl files, and start playing with the tables, they are probably the best place to start learning to mod on, because they don't need models or anything. and as long as you keep a backup, you can learn by trial and error with less frustration. You'll need to understand the .tbl files to add new content anyway.
