You take the video you want to use as a background (A) and the other video you want to be placed on top of the background (B) and put them both on the timeline. Once they've been adjusted correctly in size to one another, use the ultra key on the B video and use the drop down of the effects to remove the green from it. The key colour should be changed to a green colour that matches the green screen used with the tolerance and other drop down options altered until the colour of the green screen has disappeared from video A.