Take the two 2 1/2" x 15 1/2" strips and sew to the sides of the center you just completed. Then sew the two 2 1/2" x 19 1/2" strips to the top and bottom.
You now have this.
The top is now 19 1/2" x 19 1/2", you could stop and quilt and bind at this point or go on.