Use a flexible measuring tape

With your palm open and facing up, wrap the measuring tape around your wrist, just above your wrist bone. Note down measurement of your wrist size.

Use a string

With your palm open and facing up, wrap the string or strip of paper around your wrist, just above your wrist bone. Mark where the ends meet with a pen or a marker. Next, lay the string or piece of paper flat and measure it to where you've made the mark to find your wrist size and note it down.