Putty the insides of the windows for a more finished look. Glue picture frames on the outside of the windows to finish it.
To make the roof, cut 1” x 12” in two pieces, one at 24” long, one at 23 ¼” long. Form these planks into a roof and secure them with screws. You can use some glue at the butt-joint to make it more secure. Remember to pre-drill pilot holes before screwing in. Next, prime and paint the roof.
Then, assemble the IKEA BILLY bookcase. Screw the roof onto the top of the bookcase from the inside corners of the bookcase.
For the back panel, I used beadboard. (You could also use MDF). Lay the BILLY bookcase with roof on top of the beadboard and trace out the entire shape. Next, cut along the outline. Then, affix the board to the back of the BILLY.