In every stage, you get a medal based off the score you receive in two ways:
1. Collecting as many Coins as possible by exploring the stages and finding them.
2. Get rewarded a few of them in the end of the stage based off how fast you completed it.
For Platinum, you'll need 8000 Points or more.
Warning: This game is pretty glitchy in terms of the achievements, most of the time, you'll only get 1 achievement after you beat a stage, even though a lot of times, you'll should have gotten multiple at once. So i don't recommend going for a lot of achievements until you get a good chunk of the easy ones, but for the Absolute Perfection! Achievement, you'll probably only get it after replaying the levels a ton.
If you're having trouble with the achievements popping, beat the Daycare Dash level as Buzz and it will force the achievements to pop (for whatever reason), you'll need to unlock Buzz in Daycare Dash first, so i recommend getting your first platinum in Daycare Dash, as you'll most likely unlock Buzz in the process, which will be quite useful for any achievements that refuse to pop.