When you have the parts cut out, fold down the toothed tabs on the side strips by bending the toothed section along a straight edge so that the toothed tabs are facing at a 90 degree angle away from the side strip (towards the uncolored side).
Take one of the side strips and put the end at the top centre of the top of a pumpkin (choose one with or without a face, this will be the top of the box). Measure the strip against the underside of the pumpkin top, to determine how much overlap you need on the side strip. Glue one end of the side strip to the other end of the strip with the correct overlap to fit your pumpkin top. Fit the second side strip inside the first one, and glue it to a size that slides easily in and out of the first oval strip.