In order to sew accurate half square triangles and flying geese, it’s important to start with a well marked square. I place my pen in the corner and align the ruler next to it. This puts the ruler in the right spot, so that the line goes to the corner and not next to it.
I draw the line starting in the middle and going out. This keeps the fabric from bunching. Draw slowly and carefully.
Perfect! But if it’s not, don’t be afraid to erase and try again.