Use a flexible measuring tape
Wrap it around your wrist just below the wrist bone
Keep the tape snug but not tight
Read the measurement where the tape overlaps
Record the measurement in inches or centimeters
If you do not have a measuring tape, use a string or strip of paper
Mark where the string overlaps
Measure the marked length with a ruler
Measure the wrist on the hand you plan to wear the item on
Measure twice for accuracy