You shouldn't have a problem with it being square since you already squares all the individual pieces. Just butt up your seams as you sew everything together, and make sure all the edges, corners and seams are straight. When it's all together, fold in half and match up the corners exactly. Pick it up and it should just fall right in half smoothly. If not, use a large square ruler to square up the 4 corners individually. Then it will be square!! :-)