2. Add the walls.
When you're adding the walls, make sure to add plenty of icing on either side of it to make sure it sticks. Don't worry if it's not looking too pretty because it can be smoothed out later.
3. The roof.
Next you'll want to make a flat surface to cover up the top. You do this by sticking two more graham crackers on top, with loads of icing. You should have something now that looks box-shaped. Next, stick on two more un-divided crackers together to form the roof. Ice it all to make it stay, and don't be afraid to shove in some marshmallows for support.