If you immortalize your mare before breeding, great! If not, they will lose health steadily throughout the breeding process, so be prepared with a black orchid. You may need two before the process is over, depending on your luck.

At 30 years of age(No more, no less), equip your mare with a wand and an arrow and cover her(The stud must also be equipped with an Iris Coat). Immediately after covering, use 6 pieces of cloud to age her back to 28 years. You will be able to get her back to 28 with 6 PoC ONLY if you equip her with an arrow before de-aging, as it will allow her to age backwards 4 months at a time instead of 2.

These steps can be followed to the letter 3 times. For the fourth and final covering, you will be down to 2 pieces of cloud, so the arrow is no longer needed at this stage for de-aging. Of course, you can still equip it to speed up the gestation if you desire.

NOTE: Individual pieces of cloud won't work for this operation, as they cannot be applied to horses 30 and older. If you use a Cloud pack, it must be applied before 30.

As you've probably surmised, this process is pricey. Only one month of a Pegasus subscription is 300 passes. The BMIs total up to 1,010 if you use a Philosopher's Stone, 710-900 with Black Orchids. My advice is to use the item exchange as much as possible. My personal experience has been that it can literally cut the cost down to 500 passes per horse, which is for the Cloud Pack. If you're breeding multiple mares, that's huge!

As a final note, one of the obstacles I had to overcome early on was a lack of Iris Coated studs. Now, I have Arabian studs(Including a unicorn), so if you need a covering, just shoot me a message 

Hope this is helpful to someone out there  If you have any additional questions, feel free to PM me for my free advice, such that it is  Happy playing!