Greenscreen Help

so, I'm making this movie where I want to input someone into a clip of another movie. The problem is that I need him to be behind the people in the clip.

Is there a way to have "frames" like in Photoshop, where I can choose that the people in the actual clip are in front of my actor, and the background behind him?

I know I've seen people do it on YTMND, but I don't know how theyve done it.

lulz!

if you need telling how to do it then theres no fucking point since you'll be lost on step 2.

gb2photoshoop