Pokémon Sword and Shield feature some questionable evolution methods, including the way in which trainers get Galarian Yamask to evolve into its regional evolution, Runerigus.
To evolve a Galarian Yamask, you must be far enough in the game to access a section of the Wild Area, which will unlock after having three badges.
First, you’ll need to make sure that your Yamask has taken 49HP damage and you haven’t healed it at all or taken it to a Pokémon Center. You can have it fight some of the nearby Pokémon in the Wild Area if you aren’t quite there yet to save you from finding a trainer or farming for the right amount.
Then, once it’s taken this damage, you simply need to walk under the stone sculpture in Dusty Bowl (the same one that is on the Town Map for the area) and it’ll begin to evolve almost immediately regardless of its level.
The ideal scenario would be for you to equip your Galar Yamask with a Focus Sash and then Pokédoll away from the fight once it has used it before running to the area.
And that’s it. The Runerigus will then be yours.