Make the doorframe and lock the model. Start with a U.S. dollar bill, or other 3 x 7 paper. Valley fold and unfold along the centerline. Valley fold the bottom edge up, about twice as far as the right edge was folded in. Unfold to Step 3.
Valley fold along the mid-line of the original paper.
Valley fold the corner at a 45 degree angle. Unfold to Step 3. Valley fold the edge perpendicular to the rest of the paper. Collapse the model, keeping the extra paper locked away, as shown in diagram 12. Lock the edge by valley folding through all hidden layers.