There are two kinds of clamps - adjustable and non-adjustable. If you're using nipple clamps for the first time, its best to opt for a set with adjustable screws. This allows you to slowly adjust the pressure so it never feels too hard.
There are three types of nipples, the overly sensitive nipples, the standard sensitivity nipples and the barely sensitive nipples. The way you would stimulate each type of nipple would be different.
So regardless of your nipple play preferences, always remember that open communication with your partner is the key to maximising sexual pleasure. Let's start the Nipple Revolution, leaving no nipple untouched, yearning to be stimulated. Go forth and orgasm!