go back and try to discover why your seams are coming apart. I don't believe making 1/2"{ seams is the solution. You will have real problems when putting different shapes together However, If I were doing it I would draw the finished size of the shapes on graf paper and then add the seam allowance and then measure to determine cutting. as long ss you are making squares and rectangles there should be no problem. the problem is when doing triangles and diamonds.