To unlock a Max Move in Pokémon Go, you need to have a Dynamax Pokémon. These special Pokémon appear in Power Spots or Max Battles. Once you have a Dynamax Pokémon, go to its profile and tap the Dynamax symbol to reveal the Max Moves section. Initially, only the Max Strike move is unlocked. To unlock additional Max Moves such as Max Guard (a defensive shield) and Max Spirit (healing move), you need to collect resources: specifically, 400 Max Particles and 50 candies for that Pokémon. These resources can be gathered by visiting Power Spots, winning Max Battles, or catching more of that Pokémon species. After collecting the required resources, you can "Train" your Pokémon to unlock the new Max Moves. Max Moves can be upgraded further by spending more Max Particles, candies, and XL candy, increasing their effectiveness, but unlocking the first additional Max Move requires the resource amounts mentioned above. Summary steps:

• Catch or obtain a Dynamax Pokémon at Power Spots or Max Battles.
• Open the Pokémon's profile and tap the Dynamax symbol to see Max Moves.
• Max Strike is available by default.
• Unlock Max Guard and Max Spirit by collecting 400 Max Particles and 50 candies and tapping "Train" on the locked moves.
• Upgrade the moves with more resources to increase power.

This process completes the unlocking of Max Moves, allowing their use in battle.