Question

In Black/White 2 how do you evolve Cleffa?

Comments

Through love : http://www.serebii.net/pokedex-bw/173.shtml

Answer 1

Cleffa evolves through happiness. So all you have to do is get its happiness up a lot and it'll eventually evolve.

There are several ways to raise a Pokémon's happiness. Just winning battles with it will raise happiness.
For info on raising happiness, you can read about that here

Answer 2

Cleffa evolves into Clefairy by leveling it up at Max Happiness.
Source: http://pokemondb.net/pokedex/cleffa