Decals can be adjusted as they have a water film between the decal and its intended surface. SO, you can float it around to get the placement & alignment you want.
Stickers on the other hand, tend to grab as soon as they contact a surface. But they can be made to 'float', to get a perfect placement & alignment. First spray the area with Windex, where you want to put the sticker / decal. Place the sticker on the Windex wetted surface, but don't press down. Now, slide the sticker around, as it' should be 'floating' on the Windex layer. Once in place, press to adhere, and then squeegee out the Windex from between the sticker and the intended sticking surface. Let sit for 24hrs. the Windex should then evaporate leaving the sticker stuck..