It can be used by sending out a compatible Pokémon and right-clicking the Pokémon with the Leaf Stone. The Pokémon will evolve into the corresponding evolution.
A Leaf Stone is crafted with nine Leaf Stone shards. Leaf Stone shards can be obtained from Leaf Stone ore, found within the stone directly under leaves in forest biomes. Leaf Stone shards can also be obtained by using Forage on dirt or grass blocks with a Grass-type Pokémon.