The Amulet coin is an essential item in Pokémon Sword and Shield if you're looking to make a quick buck. The Amulet coin can help you earn more money by doubling your winnings by fighting with other trainers. All you need to do is have a Pokémon holding the coin, so make sure you use it in all your fights against another trainer. The Amulet coin does not work on wild Pokémon.
The best place to pick up the Amulet coin is just outside Motostoke, on the outskirts of Motostoke, near the Galar No. 2. Here you will find a blue sign when you exit the mines and behind the sign you will find the Coin Amulet in a Poké Ball. Collect it, then go into your Bag and equip the Amulet Coin to a Pokémon on your team. It can be a good idea to have a specific Pokémon to grow money. Whenever you face another Trainer, use that Pokémon at some point in the fight and your winnings will double.
Doubling your prize winnings for each Trainer match is a great way to earn more money, and you'll be able to afford more Poké Balls, Potions, and Revives than you know what to do. If you wish to take advantage of the Amulet coin, you can pair it with the Gigantamax Meowth which you can obtain through Mystery Gifts, then use skills like Pay Day and its Dynamax Gold Rush to earn even more money.
Remember, you have to use the Pokémon holding the Amulet coin to get double the money to win, so equip it on a versatile and sturdy Pokémon that you use a lot. If you run into awkward matchups, you can always pass the Amulet coin to another Pokémon.