Of course you can. For best results:
(1) The ring or bracelet should not be loose – it must be tight to the skin. If the molding material can get between your skin and your jewelry then it will tear the mold.
(2) Simpler, smoother and engraved rings work the best. Other jewelry may not always turn out as they can tear the mold when the hand is removed.
(3) Be extra careful when removing the ring hand from the mold. You want to move sideways to remove the ring from the mold before pulling the hand upwards and out of the mold.
(4) Unless your watch is waterproof – please remove it.