Place one foot on a sheet of paper on a flat, hard surface (e.g. the floor). You can help yourself with a ruler or by resting your heel against a wall.
Using a fine-tipped pencil, trace the outline of the heel and the longest toe (whether it's the big toe or another toe) on the paper, making sure to keep the stroke perpendicular to the surface.
With the help of a ruler, draw a line that connects a point in the middle of the longest toe, with a point in the middle of the heel. Measure the length and compare it with the measurements in the size chart.