the trick is to remember that those triangle should have the straight of grain on the outside of each corner... you do NOT want the bias edge to be on the outside of the block... so the diagonal must be the same size as the square you want to sew it to... the easiest way to get these is not to cut triangles, but to find the right size square (above site will help) and then cut 2 squares that size, slice both in half diagonally and then 'deal' them around the block like cards... the size you need is 13 (half inch for trimming) times .707.... it will be a smaller square than your block because the diagonal of a sq is larger than the square... sew all 4 corners on and then square up...