How do you get Eevee to evolve with the icy rock?

Eevee evolves into Glaceon when exposed to an Ice Rock. To do this, you need to have an Eevee in your party and then use the Ice Rock on it. The exact method varies slightly depending on the Pokémon game you're playing (e.g., in some games you might need to interact with the Ice Rock in a specific location). Check your game's instructions for specifics.