Special items in Pokémon Sword and Shield make your life much easier. Several of them are useful because they allow certain Pokémon to achieve certain evolutions or improve their statistics in combat. Reap Cloth is one of the items you need to evolve Pokémon, and Choice Handkerchief increases Pokémon's combat stats during combat.
To begin with, you must receive an old letter from a little girl. You can find the individual in question in Hammerlocke. When you get to the entrance to the city, turn right and go down the path to a staircase on the left. Go up, then go left. You should see a little girl standing next to the big building. Talk to her and she says she has a love letter to give to someone from Ballonlea.
Make your way to Ballonlea, and when you get to town, head to the left of the town's Pokémon center and go up a small staircase. You want the first house immediately after the stairs. Go inside and look for the older gentleman with blond hair. Talk to him and he will tell you how he was a childhood friend to the person who gave you the letter. This quest is how you receive the Choice Handkerchief.
The Choice Handkerchief is a useful item because it increases the Pokémon's rotation speed. However, this prevents them from using only the first move they make in a battle. So if you have a Pokemon carrying this item, use it for the first time in a battle, that's all it can do until the end. You have to think carefully about the movement you want them to use.
You're not done yet, however. After receiving the Choice Handkerchief, you must return to Hammerlocke and return to the place where you spoke to the little girl and started the quest. When you come back, you see that she is no longer there. There is no spark 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 piece of Reaper scarf after cleaning that spot long enough.
You must give the Reap Cloth to Teraclope before trading them with a friend to make him evolve into Nightclub.