Where to Find Reaper Cloth and Choice Scarf in Pokémon Sword and Shield
Particular items in Pokémon Sword and Shield make your life significantly easier. Several of them are useful because they allow certain Pokémon to attain certain evolutions or improve their statistics during combat. The Reaper Cloth is one of the items you need to evolve Pokémon, and the Choice Scarf increases Pokémon combat stats during a fight.
To start, you need to receive an Old Letter from a little girl. You can find the individual in question in Hammerlocke. When you first arrive at the entrance of the city, go to the right and proceed down the path until you reach a set of stairs on the left. Go up, and then go to the left. You should see a little girl standing next to the large building. Speak to her, and she says she has a love letter for you to deliver to someone in Ballonlea.
Make your way to Ballonlea, and when you arrive in the city, go to the left of the town's Pokémon Center and up a small flight of stairs. You want the first house immediately after the stairs. Go inside and look for the older gentleman in blonde hair. Speak to him, and he'll tell you about how he was childhood friends with the individual who handed you the letter. This quest is how you receive the Choice Scarf.
The Choice Scarf is a useful item because it increases the Pokémon's speed stat. However, it restricts them to only use the first move they do in a battle. So if you have a Pokémon holding this item use tackle for the first their attack during a battle, that's all they can do until the end of it. You have to think carefully about what move you want them to use.
You're not done yet, though. After you receive the Choice Scarf, you need to return to Hammerlocke and return to where you spoke to the little girl and started the quest. When you return, you see she's no longer there. There's no sparkle on the ground near her, either. You need to approach where she was standing and spam the A button. Eventually, you should pick up a scrap of Reaper Scarf after scouring this location for long enough.
You need to give the Reaper Cloth to Dusclops before trading them to a friend to have it evolve into a Dusknoir.