How Do I Make My Pokemon Happy In Platinum?

1:give it its fav poffins
2:win battles with it
3:win cool/cute contests
4:never let it faint for 4 days straight
5:give it moomoo milk
6:let it hold the soothe bell
7:if it was caught in the luxury ball it acts just like the soothe bell
8:go to the place in jubilife to take its picture
9:go to veilstone and go under the gym (go down to the first house) and talk to the girl with blonde hair she will give your pokemon a message
10!!!:go to veilstone market and go up ONE escalator and buy vitamins feed them to you pkmn and BOOM INSTANT HAPPINESS

Give them a puffin that they like.

Battle with it a lot.

You must feed them poffins and always make sure in battle they don't die

In order to boost it's happiness, you can just carry it in your party.
Don't ever let it faint, have it win battles, and just to make it
faster make that pokemon hold a soothe bell. A soothe bell helps boost
it's friendship with you.