With a tape measure : Measure your wrist SNUG WITHOUT SLACK, to determine your size wrist in inches or centimeters (see photo below). This measurement allows me to create a bracelet with slack in it, so that it comfortably fits your wrist.
Don't have a measuring tape ? Use a piece of string or a ribbon to measure your wrist SNUG WITHOUT SLACK and lay the string back out on a ruler to get your measurement in inches or centimeters.
On the photo below the wrist size is 16cm, size M