You have to use a very small aperture so that both the foreground picture and the background are both in focus. You also have to either turn up your iso or your shutter speed so that the picture is bright enough. Then you just hold up an old picture and line it up and take a new one
hahah it was really hard lining it up i had to get it at just the right angle. I think also having a similar lens to what was originally used is necessary as well. You're welcome
Ralf