I would get a huge piece of paper (wrapping paper). I would make sure it is square. Then fold it in half, fold in half again and half again to form a triangle. Unfold it and it should have eight crease marks dividing the paper into 8 equal sections. Measure out with a ruler from the center point the required distance for your triangle and mark a dot. Draw straight lines from dot to dots and you should have an octagon. Cut out a section for your cutting template but make sure to add your seam allowance.