Take two boards of the 18-by-24 inch pieces of lumber and align two of the 18-inch sides together. This should form an "L" shape. Do not let the boards overhang and make sure that the 2-inch edge of the boards is flush.
Attach the boards with the nails. Hammer in a nail about every 3 inches along the seam of the board. Repeat the first and second step with the other two 18-by-24 sheets of lumber. You should have two L-shaped sections.
Take the two L-shaped boards and align them to make a square. Make sure the edges are even with each other and hammer in the nails about every 3 inches to ensure stability.
Draw a circle in the center of one of the sides with a pencil. This will serve as the opening for the birds to be able to enter the house. Drill out the hole using the drill bit. The hole should be 3 to 4 inches in diameter.
Place one of the 24-by-24 inch boards on top of the opening of the square birdhouse. Attach it with nails. Hammer a nail about every 3 inches along the four sides to ensure that the roof is secure. Turn the birdhouse over and repeat this step to complete the birdhouse.