The Snapfeet Sheet.
As a first step you need to print the Snapfeet Sheet. You can download and print one sheet of paper in the A3 version, or two sheets of paper in the A4 version and stick them together with adhesive tape.
When printing, do not tick the “Fit to Page” option, and print at 100% scale. Check with a ruler that the graduated line on the Snapfeet Sheet is exactly 10cm long.