You'd start with a "base" Pokemon that'll take traits from a chosen breeding partner, or "donor", from the same egg group (baby Pokemon take on the egg group of their parent(s).
Let's use these two handsome gentleman as an example. As the base, Drampa would take the following traits from Aggron:
-A type to replace one of his current ones
-5 moves from Aggron's movepool
-One of Aggron's abilities
-Average out one of Aggron's base stats with his corresponding stat (both mon's defense in this case/2 rounded down)
-Some sort of fitting suffix to denote the newly made alternate form
-A dex entry
In the end, it'll look something like this:
Drampa-Quarry (Drampa + Aggron)
Stats: 78 / 60 / 132 (Defense stats averaged) / 135 / 91 / 36 (532 BST)
New Moves: Earth Power, Flash Cannon, Rock Polish, Stealth Rock, Taunt
Abilities: Berserk, Sturdy, Sap Sipper
Dex Entry: idk not feeling creative rn, just make something cool and concise for yours
Other submission rules include avoiding the use of previously used bases AND donors, as well as a max of 3 submissions at once. Keep your subs balanced in mind for OU's power level: Unlike Fusion Evolution this'll be a relatively small expansion on the OU meta rather than a several-hundred-mon behemoth that's ridiculously difficult to keep track of, probably 20-30 new mons or so with 4-5 winners per slate. In addition, don't mix mons that can only be the same gender as each other (eg no gallade + another mon in its egg group that's male only)
Anyways, have a submission template:
Pokemon Name: (X + Y)
While it's impossible to have a banlist expanded upon the current OU one, all Pokemon tiered higher than RUBL + Ditto are banned as bases
Ditto gives everything Impostor and the UU+ ban is to prevent subs from devolving into "make an already good Pokemon even better". As for donors, Ditto is also banned for the same reason it's banned as a base, and Smeargle is banned because even with its crappy stats, Sketch along with access to the entire Field egg group would get out of hand real quickly.
V2 of this format will be based around Regional Dex OU with Dynamaxing banned. Only Pokemon in the Galar Pokedex + eligible Foreign Pokemon (i.e. the remaining Kanto starters and Alola starters) can be used as bases, but anything (including National Dex mons) can be used as a donor. For the sake of accelerated meta development, all of the aforementioned foreign Pokemon will be themselves legal albeit only with their Galar learnsets.
Galar Pokedex: https://www.serebii.net/swordshield/galarpokedex.shtml