Whenever you want to evolve a Pokémon in Pokémon Go, you have to use candy instead of leveling it up. You earn candy by capturing the Pokémon in the wild and then transferring it, or by walking around with the Pokémon as your buddy. Some Pokémon also comes with an additional requirement, such as needing to have a unique evolution item or completing a quest before you can give that Pokémon the candy to evolve. For Galarian Slowpoke, you’ll need to finish a quest before you can give it any candy to evolve it into Galarian Slowbro.
The requirement for Galarian Slowpoke’s evolution is to capture 30 Poison-type Pokémon. Before you do this, you want to make Galarian Slowpoke your buddy. Any Poison-type Pokémon you capture while Galarian Slowpoke is not your buddy will not count towards the total. You need to make sure the Pokémon you want to evolve is your buddy.
After you’ve captured all 30 Poison-type Pokémon, you can go to the Galarian Slowpoke’s page in your collection and evolve it. You do need to make sure you have 50 Slowpoke candies available, though. There’s no chance for Galarian Slowpoke to evolve into a Galarian Slowking because Galarian Slowbro and Galarian Slowking will have different quest requirements.
Once you have your Galarian Slowking, you’re free to start using that Pokémon, and you can remove it as your buddy. Galarian Slowking is a Poison and Psychic-type Pokémon, making it a deadly combination.