Below are the different generations of the Pokémon game on DS:
Generation I - Red, Blue, Green, Yellow
Generation II - Gold, Silver, Crystal
Generation III - Ruby, Sapphire, Emerald, FireRed, LeafGreen
Generation IV - Diamond, Pearl, Platinum, HeartGold, SoulSilver
Generation V - Black, White, Black 2, White 2
Generation VI - X, Y, Omega Ruby, Alpha Sapphire
Chansey is a Pokémon from the first generation of the game, and his evolution into Blissy was introduced in the second generation. Unlike most Pokémon, Chansey cannot evolve either through leveling or as a result of exchange. Instead, you will need to raise her Happiness level by performing (or avoiding) various actions.
Steps

Step 1. Bring Chansey to a second generation or older game (if needed)
Chansey will only be able to evolve into Blissy in games of the second generation and older. If you are playing Pokemon Red, Blue, Yellow, or Green, you will have to transfer Chansey to Gold, Silver, or Crystal in order for her to evolve.
You can transfer Pokémon from the first generation to the second, but not from the first or second generation to the third and older

Step 2. Understand the process of Chansey's evolution
Chansey evolves when her Happiness or Friendship reaches a certain level. After catching or exchanging a wild Chansey, her Happiness level will be equal to 70. In order for Chansey to evolve, you will have to increase her Happiness level to 220 (the maximum is 255). In every game, the process of increasing Happiness is almost the same, with only minor differences.

Step 3. Check your current Happiness score
Determining the level of Chansey's Happiness is rather difficult, since the exact number is not indicated anywhere in the game. But you can talk to a certain character and determine the level of Happiness by the phrase he said. When checking your Happiness score, make sure Chansey is the first in your group. The following phrases indicate that Chansey's Happiness is in the 200-250 range:
- Generation II: Talk to the girl outside the bike shop in Goldenrod - “I get the feeling that it really trusts you”. Likewise in the games HeartGold and SoulSilver.
- Generation III: In the city of Vendanturf, you must find a house with a female character and Pikachu inside. Talk to the woman to determine her happiness level - “It seems to be very happy. It obviously likes you a whole lot. " Likewise in the games Alpha Sapphire and Omega Ruby. At FireRed and LeafGreen, talk to Daisy in Pallet Town - “It looks very happy. I wish Rival could see this and learn something from it. "
- Generation IV: Speak to Belle at the Pokémon Fan Club in Harthome or Dr. Footstep just south of the Pastoria entrance. Diamond and Pearl - “I get the feeling that it really trusts you”. Platinum - “It's very friendly toward you. I can tell you treat it kindly.”(She really likes you. I can see that you take good care of her).
- Generation V: Talk to the girl at the Pokémon Fan Club in Itzirrus - “It is quite friendly to you! You must be a kind person! " (She is very friendly to you! You must be very kind!). In Black 2 and White 2 you can talk to Bianca - “You two sure get along great! It looks like you are having fun together! You seem bright and cheerful! " (You really get along well! It looks like the two of you are having a lot of fun! You are very smart and funny!)
- Generation VI: Talk to the Pokémon Breeder at the Pokémon Fan Club in Laverre - "It looks like you really love your Chansey and she's always there for you!"

Step 4. Put Chance in your group to increase her Happiness
To increase your Happiness score, you must put Chansey in the active group. For every 256 steps (512 in the second generation), your Happiness score increases by 1.

Step 5. Upgrade Chance
With each new level, Chansey's Happiness indicator increases by 5 units. When the Happiness indicator is high enough, as the level increases, you will begin to receive less and less Happiness points.

Step 6. Give Chansey the vitamins
Vitamins are items that increase Chansey's stats. Each such vitamin increases Happiness by a maximum of 5 units. Do not forget that the higher the level of Happiness, the slower it will grow. Vitamins include:
- Protein
- OZ growth
- Iron
- Zinc
- Calcium
- Carbohydrates
- Growth Od
- Overgrowth OD
- Rare candy

Step 7. Pamper Chancy with haircuts, brushing and massages
You can also raise Chansey's Happiness by doing some activities. They depend on the game you are playing. There is no massage in third generation games.
- Generation II: Talk to the Barber Brothers in the Goldenroth Tunnel. Daisy Oak in Pallet Town will clean your Pokémon between 3pm and 4pm.
- Generation IV: Talk to the Masseuse Girl in Veilstone. She can be found in the house in the lower left corner of the city. The spa in the Ribbon Syndicate building in the resort area significantly increases your Happiness score.
- Generation V: Talk to the female masseuse in the town of Kastelia. In the Black and White games, she can be found on the first floor of the western building on Kastelia Street. In Black 2 and White 2, she is in the building opposite the Pokémon Center.
- Generation VI: Talk to the Masseuse Girl in the house west of the Pokémon Center in Silaj City. A hairdresser was introduced at Omega Ruby and Alpha Sapphire in Movil. It is located north of the Poke Miles store.

Step 8. Catch Chance (or Happy) in Luxball
These special Poké Balls increase the Pokémon's Happiness gain while in it. Luxball can only be found in games of the third generation and older.

Step 9. Give Chance the Bell
This item increases the Happiness points of the Pokémon holding it. Like Luxball, this item has been introduced since the third generation.
You can use Lux Ball and Bell at the same time to get even more Happiness points. Each item increases the growth of Happiness by 1, while together they increase the received Happiness points by 2 units

Step 10. Don't let Chansey pass out
If during the battle Chansey is defeated, you will lose 1 unit of Happiness. Be sure to remove Chansey from the fight if she is low on health.

Step 11. Do not use healing items
This is the most difficult part of increasing your Happiness, as using healing items during combat will greatly damage your Happiness score. For example, using the Leaf of Life can reduce Happiness by as much as 20 points (depending on your current Happiness), while Healing Dust can reduce Happiness by 5-10 points.
Try to heal Pokémon only at Pokémon Hubs, as this will not affect Chansey's Happiness in any way

Step 12. Pump Chance when her Happiness reaches 220 or more
Since you cannot find out the exact indicator of Happiness, you will have to guess it based on the phrases said. When you are sure that Chansey's Happiness level is high enough, pump her in battle to start her evolution. You can also level up her using Rare Candy.