To be clear: this are only the basics of the foot measurements. To learn it properly needs a good teacher and quite a lot of practice.The infamous “trial and error” is a very bad idea in this case..
Step 1: we create an outline. Hold the pencil perpendicular to the ground.
Hold the pencil 45 degrees to the ground and draw another line. There are devices for this, but a couple of percent off will not change the result significantly.
measure the girths at least: ball, instep and the waist. You can add the short heel measurement and the toes.
These should be enough to check the last – although not good for feet with deformities and special cases.