Prostate milking is different from prostate massage. While prostate massage can definitely enhance an erection and produce a powerful orgasm, prostate milking is designed to produce ejaculate without an orgasm. While not always successful in this regard, practicing can improve your results. Just be prepared to allow your ponyboy a couple orgasms until you get the technique down.
Prostate milking involves stimulation of the prostate through the rectum without concomitant penile stimulation. Moreover, care mut be taken in the way the prostate is worked to reduce sexual stimulation.
It's always a good idea to have your ponyboy well restrained: thus will help prevent him from injuring either himself or others. Moreover, when you are massaging the prostate, your ponyboy will likely wish to have some stimulation of his penis as well. Since the goal of prostate milking is avoid an orgasm, you will want to prevent him from being able to stimulate his penis. Physical restraints will greatly help in this regard.
With your ponyboy adequately restrained, start by putting on a pair of standard exam gloves and applying some lube to the glove. Approach the ponyboy from his side and lift up his tail to expose his anus. If he is wearing a butt plug tail, remove the plug and set aside somewhere clean (you should clean the plug portion and relubricate it before re-inserting it). Insert your finger in his rectum and feel for the prostate.
The prostate will be located only about a couple inches up the rectum. It is a smallish (about the size of a walnut) structure that is rubbery in texture. Gently massage it until you see ejaculate start to come out of his penis. Go slowly and avoid touching his penis if your goal is to avoid allowing him to have an orgasm.