set your camera on a tripod. Be sure not to move it. take as many photos as you want of the subject doing what ever. Drag your photos to Photo shop. Start with the farthest subject away from the camera and drag the 2nd photo to the first one, and set your opacity to 50% so you can see the first image bleeding through the second one. use the quick select tool to select the subject in the second photo and erase it. set opacity back to 100% and you should now have both subjects in photo one. merge the two photos and start again by draging a 3rd photo (if you took one) to the merged one. repeat steps for what ever amount of pictures you took.
If your pretty new to photo shop, just google around and hopefully you will find better and more detailed instructions than the half fast one I put up. I'm sure somone might have a link to something better.