Personally I’d always use application fluid, it’s cheap insurance to be able to position it as desired.
Place it on the spoiler dry with the backing paper still in place to be sure you know where exactly it goes, I use tape to butt to the edges of the transfer paper to control up and down and when I get the side-to-side correctly positioned I make a pencil line across the transfer paper and tape. Now when you’re ready to remove the backing paper and place the decal you’ll place it exactly where you want, apply application fluid to the decal and the spoiler, place the decal, then start squeegeeing to remove the fluid (carefully)