Via Olympus 2 Man

Position a piece of paper on a hard floor next to the wall, and step on it making sure your heel is slightly touching a wall behind.

Mark the end of your big toe and the outermost part of your heel on the paper, and repeat the process with your other foot.

Measure the distance between the two markings for booth feet, add 0.5 cm to ensure they have the right amount of space, and convert on the chart above.
If your feet have two different sizes, when comparing against the size chart use the length of your larger foot.