Hello, You must place your child's foot on the paper. The heel of the foot should correspond to the back edge of the paper. A thick book, box or wall of your home can make the back border. You can put a ruler on the tip of the longest finger and draw the line from the inside of ruler. The finger to tip on the left edge of the paper gives the correct foot size in CM. Size measurement chart; 19 - 11. 8 cm, 20 - 12. 4 cm, 21 - 13 cm, 22 - 13. 7 cm, 23 - 14. 4 cm, 24 - 15 cm, 25 - 15. 7 cm, 26 - 16. 4 cm, 27 - 17 cm, 28 - 17. 7 cm, 29 - 18. 4 cm, 30 - 19 cm, 31 - 19. 7 cm, 32 - 20. 4 cm, 33 - 21 cm, 34 - 21. 7 cm, 35 - 22. It should be 4 cm.