ummmmmm its's a maya forum so you might be better of trying this one.

https://forums.creativecow.net/index.html

it is a bit tedious to search but there are good tutorials on how to composite. Like light spill; track mattes usw.
Doing a simple composite is not to difficult though. Ideally you have both movies the same size, so 720x576 in pal for example.
You now drag your original onto the timeline. Done that you drag your cg footage to the timeline as well. And drag it so it sits on top of your video footage. Thats how it basically works.

good luck