One way to use Mewtwo in Pokemon Sword and Shield is to transfer it from Pokemon Home. Players buy Shiny Pokemon if they do not capture them in the game. Players can then add Pokemon to their team and interact with them in the Pokemon camp.
As for positioning it naturally in the game, to find Mewtwo, players must participate in Max Den Raids. Search for Max Den Raids in the wilderness and throw the Wishing Pieces into them.
The generation is completely random, so it may take several attempts. It usually appears in every ten Wishing Pieces. Cheap 6 IV Shiny Pokemon you can buy if you like to play the game.