Set up: Place a piece of A4 paper on the floor, with its short side against a wall and put your foot on the paper. Your heel should be touching the wall.
Measure: Put a mark where your longest toe lands (usually the first or the second toe).
Measure again: Measure both your feet the same way and pick the longest measurement.
Add a gap: Add 0.8 to 1.2 cm to your longest measurement, that is your insole length in cm. Compare it with the size chart below.