How do you evolve your misdreavus?

Misdreavus cannot evolve in Pokémon games. It is a final evolution form and does not have any further evolutions.

However, Misdreavus can be traded for its Mega Evolution form, Mega Misdreavus, in Pokémon Omega Ruby and Alpha Sapphire. You need to obtain a Misdreavus holding a Misdreavusnite, which allows the Pokémon to Mega Evolve during battle.

So, while you can't evolve Misdreavus in the traditional sense, you can unlock its Mega Evolution form by obtaining a Misdreavusnite.