I cut a piece of black construction paper in thirds for students to use as the base to glue on their rainbow strips. I cut the rainbow strips out for the students prior to starting the project so that the the sizes would work for making the rainbow. I cut the red paper first and then did each color about 1/2 inch shorter than the previous color.
To put it together you fold the edges and apply the glue so that the folded part (like a flap) is what you are gluing onto the black paper. You need to start with the purple (or whatever color you are using for the bottom layer of your rainbow).