Now get the pen between the big and second toe and draw a nice mark at the deepest point. Do not push in too hard but record the spot firmly.
The next step is to mark the Medial and Lateral Malleolus. This is the bump that forms the inside and outside of your ankle. I am pointing at the lateral malleolus in the pic below. Carefully trace an arc that shows where your malleoli are when you are standing with legs (or lower legs) upright. You must mark the malleolus on either side of your ankle. The position of the malleali is very important to get the sandal to sit squarely on the foot.
Your final pic will look like this above.
Send your diagram to me at firstname.lastname@example.org. Send the right foot diagram only. If you feet are significantly different sizes, then send a diagram for both your right and left foot.
Should you have any other questions mail me and I'll be happy to assist.