I square up as I go. When doing HST, I cut squares of fabric 4 1/4" for a finished 3" HST. Place the 2 squares right sides together and draw a line for corner to corner. Sew 1/4 in on each side of the line and cut on the drawn line. Then square up. I wastes a little fabric but sure cuts down on the frustration when the blocks are sewn together. I did Dear Jane a few years ago. I put hash marks on the pattern pieces that would be on the outter edges of the block. On those sides I added an additional 1/4" (insurance) and trimmed the finished block to 6 1/2. Sure helped me get accurate blocks. Being careful not to stretch the pieces also helps.