Do you mean to make them square? I have a dressmaker's L-square that I use for making right angles. You might also make a template out of paper or cardboard & trace around that.

If the fabric is a plain weave, you can also tear the squares, rather than cutting - however, I was taught that some fabrics tend to pucker if stitched on truly straight grain & it's sometimes better to skew seam lines off-grain just a bit.

HTH