Using a flexible measuring tape, measure wrist just below the wrist bone.  The tape measure should be snug against the wrist, but not tight.  If you do not have a flexible tape measure you may use a piece of string or ribbon.  Mark the length and measure with a ruler.

 

The length of the bracelet will depend on the size beads used for your bracelet and the clasp.  Bigger beads require more length and a toggle clasp I usually add 1" and with a lobster clasp I add 1/2".