First of all, we're going to have to capture a Sinistea!
Where to find Sinistea?
It is found in the Glimwood Tangle. However, it's not one of the Pokémon seen in tall grass. You must wait for an exclamation mark to appear before you and head towards it to meet the Pokémon.
How to get the Cracked pot?
To evolve Sinistea into a Polteageist, you need a special object.
You have to go to Stow-on-Side and talk to the merchant on the right. Every day, he offers you good deals. You can buy the good deal of the day but also the one of the day before. When you come back every day, you will have a good chance of finding the Cracked pot.
Transforming Sinistea into a Polteageist
Now that you own the Cracked pot, all you have to do is give the object to Sinistea. You don't need to level up, it will be done by itself once the item is activated.
