a mod means modification of game files. most good mods consist of new maps (several map editors are in existence) with new characters (that need to be voiced - get a microphone and audio editor). First - you write a script and it's generally done with pen and paper. Second - you design the maps and test them. Third - you add some minor details and do some extensive testing (on different computers with different players) and fix whatever mistakes you've made. Fourth and final - you write comprehensive description, package all the files, upload it to some server and post links to it on message boards.