Place a sheet of paper on a hard floor against a wall
Stand on the paper with your heel touching the wall
Mark the tip of your longest toe
Measure the distance from the wall to the mark
Measure both feet and use the longer measurement
Add about 0.5 to 1 cm for comfort if needed
Compare the measurement to a shoe size chart for your region
Use the width measurement as well if available
Try shoes on later in the day when feet are slightly larger
Check the fit by ensuring toe room and no heel slipping