A pad of graph paper is a wonderful thing to have. Easy to get now with back to school supplies out. Make sure it is "standard" that is 4 squares per inch (there are a couple other grid sizes). 1 square is a seam allowance.
The paper thing is a great hint. If I'm doing a bunch of cutting for a project, I do make a template from my graph paper (or the graphed acrylic sheets). You don't use it for cutting, but put down and place ruler on the edge.