So you want to breed your pony...breeding in ponyplay involves two (or more) ponies and (usually) at least one trainer/owner/handler. One common scenario involves a sub/sub couple both in pony role. A trainer or breeder will then facilitate the breeding of these two ponies. Moreover, it is not uncommon for there to be multiple spectators (usually other trainers, grooms, etc.) around to observe and assist in the breeding.
While most often, the two ponies are of opposite sex, it is not unusual to breed two ponies of the same sex. Moreover, a female pony can be fitted with a strap on to use on a male pony. Thus, I will be a little vague (intentionally) here since the only invalid form of breeding is non-consensual breeding.
Some breeders like to use a realistic, anatomical replica of a bio-equine penis (generally, either a male or female pony can use this attachment to breed to another pony - the pegasus equine sheath being a special case where a male pony can wear such a replica over his own penis). If you like this idea, you can find realistic ones on those pages. If not, that's fine too. Regardless of the cock you're using the breeding procedure is generally the same.
Prepare the mare by washing her and wrapping her tail with vetwrap to prevent it getting in the way. Put her bridle on, and use the reins to lead to the breeding station.
The breeding station should be equipped with a padded holder to which you will restrain the ponygirl. Most commonly used are "I" or "T" shaped padded holders (I will assume you are using a "T" shape). If you are having trouble picturing these, the Veterinarian has a couple good sketches of human pony breeding stations: an "I" shaped breeding station for the mare, and a breeding station allowing the ponygirl's arm to remain bound behind her back.
Position the mare so that each fore hoof goes over one of the bars at the top of the "T" and use wrist restraints to attach her front hooves to the floor in front of her. Her butt should be near the end of the long arm of the "T". This will keep the mare on all fours and prevent her from collapsing or standing up. Finally, attach a wide spreader bar between the mare's rear legs and tie her reins to the wall or floor to keep her head in place.
With your mare locked in place, lead the stallion (use a stud chain if necessary) over to the breeding station. Let him sniff around and when he gets excited he will mount her. As he fucks her he will bite her neck and use his front hooves to keep her in place. He'll fuck her hard with quick and deep thrusts. When he cums you'll see his tail twitch in timing with the spasms.
When he's done, he will unmount by himself. Often, you will want to breed the ponies a couple times in the same session, so leave the mare restrained while you wait for him to get excited again. Repeat as many times as you want.