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breeding, time, dedication, effort, healthy supply of stat philters
see guide how to stat breed

but basically, you take pets- starting with at least four (as the offspring need to not be siblings or half-siblings in order to breed with eachother)
If you want certain traits, it's best to breed for those before you start stat-breeding

Once you have your traits done, each generation you want to use a Philter (Prisma is best) to raise the offspring's stats.
Having the ability to nurture boost also helps, as it will get more nurtures and help increase the stats a little more
(having an exceptional gene test kit is also very handy, as it means that you can test the eggs and see which ones have the best stats out of any given generation so you can philter those ones specifically, if you're using a large breeding pool.)

Depending on your starting stock, the amount of time it takes will vary- you can generally expect it to take around a year with a stock of middling-stat (40-60 in the base 4 stats) pets
Magical Pie supplies will also affect the amount of time

thank you.

everything Scathreoite said is good but there is one small thing i have to clarify. The minimal amount of pets neaded in any project is 3 unrelated parents adding a forth may help at the start but if you dont have enough gold to buy the essence or if for some raison you cannot find another pet to breed (rare/LG themme, specific traits ...) you can still make your project. ^-^