This page explains how to use picnics to breed pokémon in Pokémon Scarlet and Violet.
Step 1: Unlock Picnics
Picnics are unlocked after you rest in your Dorm Room in the Academy for the first time.
Step 2: Put Compatible Pokémon in Your Party
During picnics, any compatible Pokémon in your party have a chance to make an Egg, so be sure to put Pokémon in your party that you want to breed.
Step 3: Have a Picnic
To have a picnic, you need to be in one of the Province areas, not in a town or city, and you need to be on a flat area. Then press X to open the menu, and choose Picnic.
Step 4: Check the Basket
During a picnic, if any of the Pokémon in your party are compatible with each other for breeding, you can periodically check the basket on the ground next to the picnic table to see if there is a Pokémon Egg inside. If you have Egg Power active from a meal that you have eaten or a sandwich that you have made (press (right on the directional pad) to check), it will take a shorter amount of time before an egg appears.