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.
Using the wrist measurement you've just found, find the chart for your preferred bracelet type to see which bracelet size corresponds with your wrist size.