There are so many ways to make these half-square triangles. I love:
http://www.wondercutruler.com/
you can watch the instructions here as well as lots of tips and ways to cut other items. I also have stencils and, also for years sewed them on the tissue paper pads which I no longer see in stores. Wish I could loan you my Wonder Ruler. THANKS so much for reporting on the method that gave you trouble -- I will skip it!