Get the Source code first. You really can search around for this and you’ll find it, just make sure you don’t download the RTCW source code.
Now, look at the sticky topics in this forum, they describe on how to compile your code. If you can get this step done, you can at least export your modifications to dll’s.
And then it’s all up to you what you want to modify. I don’t know where everything is, I just search around till I find a decent looking snippet. However, most files show you their function trough their names.
Put tags when you change something. And at last: compile your mod to dll’s and copy them to a mod folder. Start your mod from et when you type /svpure 0 in the console first.