How to measure wrist
Wrap a flexible measuring tape just above the wrist bone, and add 1 to 2 cm.
(The added length depends on how tight or loose you like the bracelet to be)
If you don't have a tape measure handy, you can use a strip of paper and mark it where it overlaps. Then lay the strip flat to measure up to the mark using any ruler.
If buying a bracelet as a gift, you could measure the watch's bracelet of the person you are buying for.