Place a sheet of paper on a hard floor against a wall
Stand on the paper with your heel touching the wall
Keep your weight evenly on both feet
Mark the tip of your longest toe on the paper
Measure the distance from the wall to the mark
Repeat for the other foot
Use the longer measurement as your foot size
Measure in the evening for best accuracy
Wear the socks you plan to use with the shoes if needed
Compare the measurement to a shoe size chart