One of the most important aspects of mitering a quilt is to make sure you have PERFECTLY square corners. I put the corner of my 6x12" ruler on the edge to cut. The corner on rulers is true, and I've always had sharp, crisp corners since I started doing this. I mark 1/4" from the edge on both sides of that corner to make sure that mark is perfectly centered, then stitch to that mark, turn, and start my next side from that mark.