I have found using the 1/4 inch ruler and marking the seams helps to keep everything the same size. This helps when you work awhile, then put it away for a few days to longer and get it out again. A soft pencil works great for marking the seam line. Don't push the iron, let is sit on the fabric, raise it and go to the next space. If you have bias seams, they like to move or stretch so you have to be careful.
Measure and cut each square after you have pressed it to the same size. You may have cut some pieces a bit shy or others a bit too big and if all the seams are sewed exactly, you should not have that problem. So simple to draw the seams and sew on them, even for experts, they don't have a magic touch, just take the time and we have all been there, done that.
Carol J.