Insert 4 inch into the 6 inch

This is a little hard. Get the back side in and use your thumbs to force the 4" into the hole. It fits just right so you may have to squeeze the little tabs. Click it in about two or three corrugations to make sure it's not comming out. I have really strong hands, and they were sore after this, so you may need a screwdriver or something to help pry them in.