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 and Slaking will be banned as bases.
Slaking is obvious with its already high stats and donors letting it circumvent Truant, Ditto gives everything Impostor and the UU+ ban is to prevent subs from devolving into "make an already good Pokemon even better". Depending on the base the new form can have a Mega, but only if the vanilla form of that mega is also RUBL or below. 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.