I starch the fabric before cutting. Then I cut my original squares a little bigger than the directions call for. Then I mark the diagonal on the back of whichever fabric is easier to see. Some press a diagonal line into the squares instead. I sew a scant 1/4" away from both sides of the diagonal. Then I touch all those now sewn squares with a hot iron to set the seam before cutting them. Let cool a minute or two. Then finger press them open with whichever side you want the seam to go. Then I lay the hot iron on them again to press them. No squiggling around with the iron.
I will also add that I pre-wash my fabric. Some of the thinner fabrics and fabrics that have metallic woven into them seem to shrink or warp every time a hot iron hits them and no amount of starch or pre-shrinking seemed to help. Cheap muslin seems to be the main culprit it my stash now but before I started pre-washing & starching all my fabric before cutting, I had the problem a lot.