What you need – string (bale twine will do) and a tape measure.
Using a bit of string is best as it enables you to see where the rhythm beads will sit on your horse. Some people like the beads to sit a bit higher other people like them to sit a bit lower. So you can decide what looks right for your horse. Rhythm beads clip onto the mane so they do not slip down the neck if the horse puts it's head down. They are not designed to be attached to a saddle or other tack.
Take the string and place it around the neck on the horse ideally on the groove down the front of shoulder where the neck meets the shoulders and chest. And make a knot in the string to make a loop round the horse’s neck. Step back and assess. Is it too high? If so loosen the string out. Is it too low? If so shorten the string. Keep stepping back to assess, making adjustments until you are happy.