Use a flexible measuring tape
Wrap the tape around your wrist where you would wear the watch
Keep the tape snug, not tight
Note the measurement where the tape meets
If you do not have a measuring tape, use a strip of paper or string
Mark where the paper or string overlaps
Measure the marked length with a ruler
Measure in inches or centimeters
Measure your wrist before choosing case size and strap length
Recheck the measurement for accuracy