first get yourself some charm packs...yeah that's me promoting my shop.
fold in half one block (all charm packs are 5x5 so if you are just cutting out your squares cut them to this length and width)
line up your ruler at an angle (make sure the fold is towards you so when you open it up you end up having one whole piece) draw a line with a pencil down your angle. this is your cutting line cut with pinking shears like mine or use plain scissors, it will be just as cute!
now you have your triangle (extra step would be to sew around 1/4 inch in so the flag won't fray as much, but i skipped this step seeing that i'm only going to use it for birthdays or such things and won't get a ton of wear) now for this particular project i used store bought bias tape it was the easiest option.