PJ Masks: Heroes of the Night (US)

A diorama is unlocked at the completion of each mission. The type of diorama (bronze, silver, or gold) is determined by how many gems you collect in that mission. You'll need to collect all 200 gems in a mission to get its gold diorama, so you'll need to collect all 200 gems in all 16 missions for this achievement.

The gems in all 16 missions are laid out in the exact same manner:
 

• ​​​​​​The first 100 gems are during the autoscrolling section at the beginning of the mission. The car ones are easy since you can only go left and right, just make sure to hold  when you get to a ramp to go high enough to get any gems in the air. The other vehicles allow you to move up, down, left, right, or center, so you'll need to pay attention to the gems - they indicate where you need to be by the "tail" sticking out of them. Gems that are up have tails hanging down, and gems that are down have tails pointing up. It's a little awkward at first but you'll quickly get used to it. 
• The second set of 100 gems are during the actual mission. You'll play as all three characters once in every mission, and the gems are split up almost exactly per character: each character always has 32-34 gems in their section. The Gecko and Owlette ones are very easy because they're all in your path. The Cat Boy ones, especially in later levels, require you to platform around and explore a bit. Just always take either the high or low path, then double back to take the other since gems are always in both paths. 

That's all there is to this achievement. If you miss a gem, especially during the autoscrolling section, you can pause and select to restart the level. I highly recommend doing so if you miss a gem so that you don't have to replay the level all over again after finishing it.