It's really easy.. takes a week or somenthing until you understand
Basicly, you modify files (From the patch_mp.ff) for an example...
The _killstreaks.gsc controlls the killstreaks xD
People use _rank.gsc and _missions.gsc cause they have a thread called "onplayerspawned" It's basicly what it's in the name.. It controlls what happends when a player spawns... so you type things like
[php]self thread dostuffs();[/php]
And then you do like
stuffs in here
Also, if you don't wanna do things like this... You can use patch makers.
I'm not sure if I can post links... so...