To evolve Chansey successfully, Trainers need to achieve specific requirements while using them to trigger the evolution. But, for new players, I suggest you buy directly.
Get 220 Happiness. At its core, Chansey evolves into Blissey by achieving 220 Happiness with her. Succeeding in this endeavor can be done successfully through this process:
1. Get the Soothe Bell. Using this item will allow Chansey to improve its Friendship and Happiness much faster.
2. Focus on the Camp. As players know, the Camp is where they can play and cook with their Pokemon - essentially a place to increase their Happiness.
3. Level up after reaching 220 Happiness. By the time Chansey reaches 220 Happiness, she's ready to evolve. To do this, players simply need to fight until Chansey manages to level up.
If you think the above process is too complicated, then you can go to PKMBuy to buy one, or you can Buy any you want.