Place foot on a piece of paper parallel to the length of the paper
Mark the position of the back of the heel
Mark the position of the longest toe (not always the big toe)
Measure the distance between these two marks parallel to the length of the paper
Mark the widest points of the inside AND outside of the foot
Draw a line parallel to the length of the paper through on of the points
Measure the distance between this line and the other point perpendicular to the line