if you don't want to use stabilizer soak your t-shirts in starch and let them dry then iron the snot out of them. Then get a square ruler or l-shaped ruler and that will help getting them squared up properly. I would also suggest maybe getting a larger mat so that you can just turn the mat or walk around the mat as you cut. that could be part of the problem. the mat is not large enough to accommodate a 14" square. I had to get a real cutting mat when I started making quilts. I had been using a 6 1/2" x 12" cutting mat. it will make a world of difference in the cutting.