I don’t like to square up blocks anymore than I have to. It’s tedious work, and it wastes fabric. Don’t get me wrong, there are times when it is necessary. But if I don’t have to do it, I am happy not to. This method works most of the time. Just be accurate in your seam allowance. Make one or two squares first to check out your finished size. Make adjustments in your seam allowance if needed. Just one thread width can make a difference.
Cut two squares of fabric 7/8” larger than the finished size. For example, if you need 3” half-square triangles, cut your squares 3 7/8”. Draw a diagonal line through one square. Stack on top of other square, right sides together. I first pin the outside corners together. Then I pin the other two corners. When sewing a bias (or diagonal) most directions will tell you to be careful to not stretch the bias. But when you sew ¼” on both sides of the drawn line, what really happens, is that the stitch draws the fabric up and shortens the stitching line. A “not to square” square results when you cut the square apart on the drawn line.
If I find that that is happening (it happens a lot with very fine fabrics), I will use a piece of cheap tissue paper as a stabilizer under the fabric at the stitching line. Just sew right through it. It tears off easily. Cut apart on the drawn line and you will have two half-square triangles. Press to one side.