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 edge to the toe mark
Repeat for the other foot
Use the longer measurement as your foot size
Measure in centimeters or inches
Compare the measurement to a shoe size chart
Measure feet at the end of the day for best accuracy
Wear the socks you plan to use with the shoes when measuring