Instructions
Go to Route 216, Route 217, Snowpoint Temple or Lake Acuity. Bring a Pokemon that knows a damaging Fire-type move. Walk in the long grass to get a Sneasel to appear. Damage the Pokemon with Fire-type moves and capture it with an Ultra Ball.
Go to Route 224 or Victory Road. Pick up all the Item Balls, which resemble Pokeballs, in the location. One of the Item Balls will contain a Razor Claw, which is necessary to evolve Sneasel. If Sneasel isn't in your party, retrieve it from a PC at a Pokemon Center.
Go to your Bag from the Game Menu of "Pokemon Diamond." Use the D-pad to highlight Razor Claw and press "A." Choose the first option, which is "Give." Highlight and select Sneasel to give the Razor Claw to him.
Travel to any of the following routes: 214, 218, 223, 225, 228 or 230. One of the Item Balls on the route you choose will contain a Rare Candy. Continue playing "Pokemon Diamond" until 6 pm arrives. You can evolve Sneasel anytime between 6 pm and 4 am.
Go to the "Items" section of your Bag. Select the Rare Candy and choose to use it on your Sneasel. If the Sneasel is holding a Razor Claw and it's between 6 pm and 4 am, the Sneasel will evolve into a Weavile.