Pokémon Sword and Shield are getting an expansion pass, bringing two new DLCs to the game later this year. This will introduce two new areas and 200 Pokémon that are not currently in play.
Additionally, an update for the game will arrive today to celebrate the upcoming DLCs. This will introduce Galar Slowpoke and Galar Slowbro to the game and you will also have the option to add them to your Pokédex.
As soon as the update is available and you have downloaded it, you will need to visit Wedgehurst Station. A new trainer has been added to the game and you can meet him there. The new trainer will be essential during the next DLCs, so consider this your introduction to future content.
A cutscene will play, and once it's done you'll be able to catch the Galarian Slowpoke. Take it in the usual way, weakening it a little and then using a Poké Ball. Galarian Slowpoke can evolve into Galarian Slowbro, but we're still not sure what level they need to be in order to evolve yet. We will update this guide later today when we find out in the game.
You don't need to make any special purchases or pre-order the expansion pass to get the Slowpoke Galar, all you need to do is visit Wedgehurst Station and talk to the new NPC you can find after the update.