There is nothing difficult, everything you need to do is to measure your wrist circumference. Take a sartorial meter to measure it.
If you don't have this device - don't worry. Take a piece of thread, a cord or just a strip of paper and wrap it around your wrist. Please make sure that all these things are wrapped tightly while you measure your wrist.
Then measure received segment with a ruler, the result will be the length of your wrist circumference.
Please mind that your wrist measurements will influence size of your bracelet. So remember the saying: "Score twice before you cut once"