Instructions
Catch a Chansey by traveling in the grass on Route 13, Route 14 or Route 15. Chansey has a 1% encounter rate so it may take you a long time to encounter once.
Use Fighting-type Pokémon to weaken Chansey. Because it is a Normal-type Pokémon, it is weak to Fighting-type attacks. Once you have weakened the Chansey, use any Pokéball to capture it.
Place Chansey in the first position of your party. If you have any vitamins such as Iron, Protein, Calcium, Zinc, or Carbos, give them to Chansey to improve its happiness. Use it in battle, but do not let it faint. Additionally, take Chansey to the masseuse in Goldenrod City to give Chansey a massage. You can only do one massage a day. The easiest way to raise happiness is to attach a Soothe Bell to Chansey and simply ride on your bicycle back and forth. After several minutes, your Chansey should be at maximum happiness. Once you raise it one more level, it will evolve into Blissey.