Tear a piece of cardboard from the back of a tablet or something. The sides should be straight. At the one corner, measure over 5-1/4". Go to the other side of the same corner and measure 5-1/4". On the ruler you will find 5",
then if it has a short line (1/8") and then a larger line, that is the 1/4". If you cut this out and lay it on your cutting mat,
this should be correct. If it is off a little, trim it to fit. Make sure it is square. If you use a piece of fabric, use a piece from
something else, and cut a square. Fold it in half and if all lines match, it is square. I hope this helps.