Find a box suitable for building a playhouse. The first and most important factor in determining the suitability of the box is safety. Be sure the box is free of staples and that any plastic straps have been removed. Be sure that the box is clean and completely emptied of any potentially dangerous packaging materials (such as extruded polystyrene foam and plastic wrapping).
Once the box has been deemed safe, be sure the box is large enough to accommodate its future occupants and that the cardboard is in tact so the box will not collapse during play.
Seal the ends of the box. Secure the flaps with packaging tape or duct tape.
Choose which side of the box will become the front of the playhouse. This will be the wall upon which you create the door and windows to the playhouse. The front wall of the playhouse should be a non-opening wall of the box. (Typically, the ends of the box used for opening will be comprised of flaps which are not ideal for cutting through.)
Draw a door on the front wall of the playhouse with chalk or a pencil. To maintain the structure of the box, the best place for the door is in the center.
Cut the door. This step must be done by a responsible adult only. A box cutter works best for this task but scissors will work, too. Be sure to leave the "hinged" side of the door in tact. You should only cut along the top, bottom and one side of the door.
Add windows to the playhouse if the box is large enough. You can add windows to the left and right of the door as well as to the other walls. The style of the windows will depend on the skill and vision of the adult supervising the project. Simple squares cut from the cardboard walls make fine windows. You can also create a paned window by cutting squares in a four-square pattern (leaving a cross shape in the center just like a real paned glass window).