Use shortcuts! Position the corner square so when you fold it back you have extra to trim to fit the block. Starch and press the folded corner block in the exact place you want it to be, unfold and then sew on the crease line.
Put a few drops of basting glue to hold the square in perfectly in place if you want. I never exact cut the corner squares, I just make sure they are at least 1/2" bigger then I need. This is a great way to learn about leaders and enders so you will have an extra HST made for every corner to use later. Elmer's School glue will be your best friend so stock up. It's not really a glue but a very strong starch so it washes out. Heat with an iron and the glue hold the fabric together instantly. Basting glue works great too but Elmer's is so available and cheap.