Decals = stickers set
To easily setup start to let's clean the plastics. Then heat slightly with a hair dryer or with a heat gun on the side inside.
The better temperature for stickers and support is 20 / 25°. You will apply a film of soapy water on the face a paste. This will facilitate the positioning on the cover.
When the sticker is perfectly in place, a shot of hot air heat will help to apply the more rounded forms.