Lay the 4-by-4-inch piece of wood onto a flat surface.
Place one of the 4-by-6-inch pieces upright on top of one side of the 4-by-4 piece. Nail it into place.
Place another 4-by-6-inch piece upright on top of the 4-by-4 piece, opposite from the first piece. Nail it into place. These will be the sides of the open-cup birdhouse.
Place another 2-by-6 piece upright on top of the 4-by-4 piece, placing it along the back of the birdhouse. Nail it into place.
Place the remaining 2-by-6 piece upright on top of the 4-by-4 piece, placing it along the front of the birdhouse. Nail it into place.
Drill a hole in the center of the front of the open-cup birdhouse using a spade drill bit. Shake out the sawdust from inside the open-cup birdhouse before nailing it into a fencepost or tree. When you nail up the birdhouse, make sure that the opening faces out.