set up your camera on a tripod so it doesnt move while your taking the shots. make sure you dont pan the camera so the background stays the same. most nikons can shoot at 2.5 pics per second, so when you have all those images, open up photoshop. select a background image. use the lasso tool to copy and paste each person into the background frame in the right order, but set all copied images to blur on the edges. also, look on the internets.
heres a rough one i made. i panned the camera so it turned out kinda rough, but you get the idea