Once you have decided on a layout for your interior and exterior, cut the ground floor openings for your house, first cut the back opening, cutting it as an arch to help support the lid. (see photo). Before you cut the openings for the doors and windows, make sure you have allowed space for an interior run of stairs (if you want stairs) as well as marked the height of the second floor, so you can set any second floor doors or windows above the height of the floor. I suggest you leave cutting the openings on the second floor until you have the interior floor finished (see next steps). When you are sure you have everything positioned to your liking, use a sharp knife to cut the openings. Cut the openings so the doors and windows are left in one piece, you will use these curved scraps of card for other parts of your house.
When you have all the openings cut, use your sandpaper on a scrap of wood or small sanding block to sand the cut edges of the openings.