While perhaps more often associated with cow play (hucows), there's no reason why you can't incorporate lactation, simulated or otherwise, into your pony play. If you are of the mind that pony play should closely mirror bio-equine reality, yet are interested in lactation play, you're in luck: in some regions bio-mare's are regularly milked (the milk is used to make kumis - though I'm not sure I would advise using your human mare's milk for this purpose).
Whether you are engaging in simulated milking or actually trying to collect something, the procedure is the same. While I do appreciate both two and four legged ponygirls, I think milking seems more natural on a four legged ponygirl, so that is what I will describe here.
Start by restraining your ponygirl on all four legs by pinning her forearms to her upper arms and her calves to her thighs. I tend to like vetwrap for this purpose, but rope or leather cuffs are fine. Once you have made sure your ponygirl won't be standing up or trying to kick you during the milking process, tie her bridle or halter to a post or other object - you don't want her trying to turn around and bite you.
With your ponygirl properly restrained, kneel on her left side facing her tail. Gently touch her breast with your left hand and judge her reaction. If she accepts your touch, pat her neck and tell she is a good girl. If she is resistant, correct her gently but firmly (e.g. tug on the reins/lead rope, swat with a crop/hand).
Once she is comfortable with you handling her, use a sponge soaked in warm water to clear her breasts. After cleaning her, start the milking process by forming an "O" between your thumb and forefinger around her breast above the nipple. Tighten the "O" to be snug against the base of her nipple.
Slowly move the "O" down her nipple, keeping a snug contact while doing so. Just before you reach the tip of her nipple, stop and return it back to the base of the nipple. Repeat this process as many times as required.
When you are done milking her, wash off her breasts again with a lightly damp sponge before untying her lead rope and releasing her restraints.