
Step one: build the floor
Once you have a scrap lumber source, start with a base platform made from treated 2x4s. Screw or nail them together into a 20"x18" rectangle, and add a floor of plywood or MDF board. (Note: the coop is deeper than it is wide; the front of the building is 18" wide.)

Step two: attach side walls
Next, start adding the side walls by attaching 2x6s to the base with 3" screws. One wall is 36" high, and the window wall is 40" high.
Attach a support board, as shown in the pictures, to keep the boards together at the top of the walls; keep these boards 1/2" from the top for adding a shelf later. On the taller wall, leave a space for the window and the chicken door.

Source