The points going down the center have to be on the diagonal perfectly for the points to come out right. The square rulers have a line the goes diagonally though it. Place the line exactly on the line and make sure the block is the right size by carefully cutting up and over the top, turn and do the same with the other side. Don't mash it out of shape, sometimes we take off too much that way.
I want to add something ... The pressing should be done with a dry iron in a press lift motion. It is often the pressing that gets us into trouble with the bias edges of the hst's.
I hope this helps you.