If using the sled for a competition, check the rules about design specifics before making the cardboard sled because each race can have different rules.
To reduce friction, use duct tape on the top side to tape the boxes together. If the seams feel weak, fill them with craft glue for added stability.
Apply paint to the top of the sled and decorate the top as desired. Allow the paint to dry completely.
Flip the sled over and apply a thick coat of carnauba or heated paraffin wax to the sled's bottom.