there are sites that show how to mark that but i didnt' love them. i have done X's with zig zag stitches. doing one side and then going the other next, meaning zigging to the right, then left, then right and then left. then starting again but going zigging to the left first, then right and so on. if more needed than X's you can go back and move down a bit and use the already stitched lines for your measurements to keep it nice and even. Good luck.