I got my kit a couple of days ago. It looks to me like you glue the center blocks in before applying the sheeting. You do have to plane or sand the block corners off to match the coutour of the wing ribs. Mark the ribs with a strong pencil mark on the edges before you glue the blocks in. When you are sanding, stop as soon as the pencil marks start to show signs of being sanded off. This way you won't sand too deep and change the coutour of the ribs.