You may find it easier to have someone help you with this. Some like to trace an outline of the feet on paper and measure from there. It’s up to you. Most of us have feet that are slightly different, so please perform the measurements on both feet separately.
While standing on a flat surface, measure the distance from your longest toe (usually either your big toe or the next one) to your heel. Do not perform this while sitting.
Measure the distance from the rounded part made by the ball of your foot near your big toe to the widest part near your little toe.
Measure the circumference of the ball of the foot near your toes (all the way around the area measured in Item 2).
Measure the circumference of your foot nearest your ankle.