Instructions
Catch a Chansey in the Safari Zone using a Luxury Ball --- keeping a Pokemon in a Luxury Ball makes it gain Happiness more quickly.
Place Chansey in your traveling party of Pokemon so that you will have it with you at all times. Every time you walk 256 steps, Chansey will gain happiness points.
Give Chancey a "Soothe Bell" item to hold --- this will increase the amount of Happiness that Chancey gains from any event.
Gain as many experience points as you can with Chansey, either by battling or by using the Exp. Share item. Every time Chansey gains a level, it also gains happiness points.
Feed Chansey Vitamins and Effort Value Berries to improve its stats --- each time you feed it a Vitamin or an EV Berry, it will gain Happiness.
Keep walking around and leveling up Chansey until its happiness level rises above 200 to "MAX". When Chansey's Happiness level is high enough, it will evolve into Blissey.