How to Green-Screen in Adobe Premiere Pro (Using Ultra Key)
This video shows you how to green screen in Premiere Pro using Ultra Key.
Note: You don't have to use a green screen specifically, it can be of any color, so long as none of the actors wear anything even remotely near that color! (green is not good for a tree and so on) Try and pick a color that contrasts whatever you want to preserve.
Also, the green screen has to be consistent. Not different shades of the color!