If you are looking to get more Max Raid battles going on Pokémon Sword and Shield, you will need to acquire a piece of wishes. It is a useful item that you can get from seller Watts, which costs 3000 Watts to purchase. The most iconic location for seller Watts is at the entrance to Motostoke, at the bottom of the stairs in the black and white dress. When you get it, you can find it in the "Other Items" section of your bag.
You can use the piece of wishes in the wilderness of the Galar region. You need to approach an already empty lair, which should appear gray. You can't use it on bright burrows or if it has a bright red light coming out of the center.
When you find an abandoned lair, you typically can't interact with it any further. But if you have a wish piece, you have the option to lure a Dynamax Pokémon to this location. When you cast your wish piece, you get a notification to save the game. You should now see a distinct dark light shooting out of the burrow. You can face it and treat it like any other battle with a Pokémon Dynamax, where you can choose to invite other players or not.
Usually, when you approach a lair with a Pokémon Dynamax, you receive Watts from the energy. However, since you have lured Pokémon there, you won't get any Watts when you approach the lair. Regardless, you have the opportunity to fight a Dynamax Pokémon and receive some experience points to end the match.
Using the Wishing Piece consumes it and you will need to use more in the future.