HOW TO FIT A CUFF
Below is a You Tube link that has valuable information on how to put a cuff on properly.So please click on that & check it out.. I also suggest you read what I wrote below as well,since I also cover things that they do not cover..

https://youtu.be/bU6QwykAo7w

The best way to get a good fit is to measure a cuff bracelet you already have that fits you well. The interior circumference is the inside measurement WITHOUT the gap. The total circumference is the inside circumference of the bracelet from end to end INCLUDING the gap. EXAMPLE: If the inside Circumference end to end is 5" and the Gap is 1", the total Circumference would be 6". If the cuff has a 5-3/8” inside circumference & the gap has a 1-1/8” gap, then the total Circumference would be 6-1/2” wrist size. Remember if the cuff can be adjusted, the only thing that can be adjusted is the gap opening & that’s only if the gauge of the metal is light enough to adjust. The interior measurement will always be the same.

If you don't have a cuff bracelet , then measure your wrist where you want to wear the bracelet. Cut a strip of paper approximately 8" long and as wide as the bracelet you would like to wear. Wrap the paper around your wrist where you would wear the bracelet. You might secure the paper with a piece of tape to get a better feel of the fit. Remove the paper and measure from the end to the mark for the circumference of your wrist.

****The thickness of the fleshy part (above your wrist) is equally as important as to whether a cuff fits properly & even though the cuff could fit your wrist, if the gap is too small or too large, can make a huge difference in the cuff fitting. It’s important that you know what size gap opening you need for the cuff to comfortably go over the arm.****

A cuff should not 'roll' around on your wrist(Too loose),nor should you see flesh hanging over the edge(Too tight) Always put the cuff on above the wrist bone where it is fleshy,then bring it down to wrist.And NEVER "squeeze" to tighten!! You will lose the integrity of the metal & if there are stones,they can loosen or pop out. Rather, turn your wrist over,push down on one end at the terminal,(gap) push down on the other at the terminal.. Go easy,if the gauge is lighter, since you can bend the metal out of shape. With know how, adjusting can be done either by yourself or your jeweler,depending on the thickness of the gauge.