Step 1: Requirements
A tripod (a cheap one's fine)
A digital camera that can be attached to a tripod
I know what you're thinking. Paint. I know, it's bad. Just give it a try. It does everything that you'll need to do to make this look good, primarily, dropping in the other picture of you.
Step 2: Thinking of an Idea
Step 3: Taking the Pictures
Now, take the first picture. You want to take a picture of what one of you is doing in the picture. To take the picture, get someone else to take it, set the camera on a timer, or use a remote.
Now, to take the other pictures. Make sure that you do not move the camera or tripod at all. Even a slight movement will mess the whole picture up. Take the rest of your pictures of doing what you're doing in the picture.