How do I evolve my Sneasel?

[KingXIII]

Topic title.

[TheBirdLord]

It can evolve to Weavile if it is leveled up while holding a Razor Claw at Nighttime.

If you need any more information on wild locations or how/when pokemon can evolve, you can check their pokedex entry in-game.
(You don't need the mon to be registered to your dex for this)

[Gryf]

which hours are nighttime

[RoiZzz]

Under your money, there is a sun or moon symbol, when one of the two is showing and you know what time the game is in, in the case of Sneasel you need to equip the item Razor claw and get a level with it, you can be defeating npc or wild pokemon ... even a rare candy.
 

as long as it is in night status.