Xbox 360 gameplay recording general question?

so lets say i record me playing xbox 360. I got it all on my dvd rw disc BUT that only records the gameplay and the sound within the game.. how do i inject my voice onto this? how do you edit your videos when you have finished copies. I watch some gameplay videos on youtube and there chatting whilst going through the video (like playing). How do i do that? Dont send me youtube videos as i have searched alot of them. Thanks.