The easiest way for a woman to use a vibrator on a man is simply to stroke his cock and balls with it. She can run the vibrator along the length of his shaft and around his cock head. The base of the glans and the underside of the penis head are particularly sensitive spots.
Another way to stimulate a male partner is to stroke his balls with it or to hold it directly under the scrotum. Pressing the vibrator against the perineum (the spot of skin between the balls and the asshole) can also be very pleasurable. If you press into this spot, the vibrations will also transmit to the prostate.
If the vibrator is an insertable dildo or probe, it can be inserted in the man's ass to stimulate his prostate directly. To hit the prostate, insert the vibrator two to three inches into the ass and aim the tip toward the penis. Make small in and out stroking motions with the vibrator rubbing against the prostate. Prostate stimulation combined with vibration can produce very intense male orgasms and sometimes increased ejaculation. Whenever inserting anything anally, use a lot of
lube, go slow, and don't force it. Also, only use toys for anal insertion that have a flange or flared base, or something at the end to keep them from going too far into the ass. If you insert a vibrator without a base too far into your partner's butt and lose your grip on it, you may not be able to get it out.
If a woman is using her own vibrator on a man, she should take certain hygiene and safe sex precautions. Using a vibrator on a partner and then using it on yourself without cleaning it first is unsanitary and can be the equivalent of having unprotected sex. If you get a man's pre-cum or semen on your vibrator and insert it in your vagina, you run the risk of pregnancy or getting an STD. If you use a vibrator on a partner's ass, you need to clean it thoroughly before using it on your pussy or inserting it vaginally to avoid the risk of a yeast or bacterial infection.