well, first import the clip and the background footage or clip into premiere, then put the clip on lets say track 2 on the timline, and put ur background footage on track 1, now, go to the video effects folder, and then go to the keying folder, in their, there is a key called green screen, or something like that, apply that key to your clip. if the green screen doesn't look that good, ex: u can still see the green screen a little visible, or the person in video starts to get invisible type, then go to the filter properties and adjust the green screen key. there is also another key in the keying folder whcih removes anycolor u want from the screen, so u can put it to remove only green color, i can't remember the name of that key, find it it premiere, experiment with it. i hope that helps!