This is a type of Nuzlocké and all rules for that apply here but I’ll assume you know those
Someone (like a friend or maybe random people in chat) will receive your random encounters in exchange for eggs as soon as trading is available, which, in USUM, is after visiting the Pokémon Center in Hau’oli Shopping District. You just do a regular Nuzlocké beforehand since there isn’t anything you can do. You are to hatch at least one of these eggs as soon as possible so you have a hatched Pokémon on your team, and then trade away your starter for an egg.
There is no preparation beforehand other than getting a friend to have the eggs.