The receiver should have a bowel movement beforehand, if necessary. Both partners may prefer if he has a series of enemas shortly before, injecting only enough water to rinse out the rectum. He shouldn't take too much water too deeply, as it may come out later at an importune moment. Warm water enemas not only eliminate traces of feces, but they also relax the anal muscles and stimulate the whole are. Wash afterwards with warm water and gentle soap.
Start with foreplay - engaging in sexy talk, massage his buttocks, have him expose his anus, give him analingus, spank him. Circle the sphincter muscle with a finger and so on. If you delay the moment of insertion and tease him physically and verbally, he'll reach the point where he may want it so badly that he'll beg.
Try stroking the perineum (the external area of skin between his anus and his balls) while your finger is inside his anus. This will simultaneously stimulate the prostate from a second angle.
Oral sex along with a prostate massage can feel incredible. Suck his penis in rhythm with your finger in his anus. Don't forget his testicles. Lick and gently suck on them too! Wrap your thumb and forefinger around the scrotum, so it encircles it, then gentle pull while sucking his cock.
Maybe give his prostate a break and just finger his anus or just inside it. That's pleasurable in itself. He might prefer that. . . to have a finger slide in and out.
He may like to have you insert two or more fingers at once. Start by twisting your index and middle fingers, which will make them enter more easily. You may also use lubricant or saliva to help slide it into the anus. Experiment with ways of gently stretching the anal sphincter. Once he is really open, you can insert a butt plug, dildo, vibrator or prostate massager.
When using lubricants, use water-based, like KY Jelly, as they don't stain fabrics like silicone-based lubes and are easier to clean up afterwards. But you have to keep applying, because it tends to dry out rather quickly.
Enjoy the process of finger penetration. You shouldn't race to the prostate. Give the anus attention and treat it gently and with patience. It feels wonderful having a lubricated finger slowly plunging in and out and it can be fascinating for the person penetrating the other.
The prostate is located about 2-3 inches inside the anus toward the front of the body. Often one can feel it through the wall of the anus as a firm rounded mass about the size of a quarter. Try circling it with the pad of a fingertip around the edge of the prostate. Apply only gentle pressure, like when you rub your own eyeball. You can also use repetitive tapping motion or sliding your fingertip back and forth over the prostate. Have fun. . . experiment.
Beware however, if you aggressively rub the prostate with a lot of pressure, it may feel good at the time (especially if you're under the effects of a PNP party favor), but you'll cause it some injury and pay for it later. Hours afterward, it will swell and become sore. It will become difficult to pass bowel movements for several days, until the swelling goes away, like a bruise. So, as a rule, apply some pressure, but don't overdo it. Experiment.
One nice trick is to insert a finger using an edible lubricant or lots of saliva and then lick his anus around your finger. This can make an exquisite surprise having his anus fingered and licked at the same time.
Orgasms from prostate massage and anal fingering can be incredible strong, especially for a first timer. Don't be surprised if he makes noise while you finger him or groans out in joy when he cums. He may ejaculate more semen than ever before.
Many men prefer a partner to withdraw the finger during orgasm. Somehow it feels right, being able to concentrate on orgasm and ejaculation.