The measurement is very important and has to be carried out in both legs separately. There is a related video on how measurements are exactly taken.
Place a sheet of A4 paper on the floor and then take the imprint (outline) of the foot. Measure the area of the ball girth and instep girth. Moreover measure the exact length of the foot. During the measurement process, we consider any malformations or foot problems (e.g calluses, etc) with the use of a specific platform (Plantar pressure device) while gait analysis is made.