Parts 1 & 2 can be found here
Part 3: The Flap
Put interior & exterior fabrics right side together. Round off the bottom corners (flap will be 15" wide by 13" tall). I used a CD to round the corners, but a plate or glass will work too.
Sew around 3 sides and clip the corner.
Turn right side out and press!
Topstitch and then press again.