The first step is obviously to find an Applin. It is one of the Pokémon that can't be seen in the grass but for which you have to wait for an Exclamation Point.
Where to find Applin?
Go to Route 5 and run through the grass until you see an exclamation mark appear. Its spawn rate is low but you will have a chance to run into a Applin and capture it.
How to evolve Applin?
Whether you have a Sword or Shield, there is an NPC giving you the object to make your Applin evolve.
To find this NPC, go to Hammerlock. Head to the far left of the city.
At the end, after the Pokémon Center, you will find a mini park just before a stone arch (symbolizing the end of the city) and access to Route 6.
The man stands in front of this exit. He will ask you if you have a Applin on you. Don't worry, even if you give it to him, he'll give it back to you!
As a bonus, you will get one of the two objects (depending on your version), to make it evolve!
