For larger surfaces to be covered, it’s always helpful to spray the surface with a little water before applying the decals. You can also add a few drops of dish soap to the water to make it more slippery. This allows you to make adjustments while applying the sticker without damaging it. For stickers on a backing sheet, you can use a plastic squeegee to push the water out from under the sticker by running the squeegee diagonally across the surface with light pressure. This prevents pockets of water from forming under the sticker—which would take a long time to dry—and the scraping action further improves the bond between the sticker and the surface.
Stickers without a backing film can also be applied this way using a clear document sleeve from an office supply store. Of course, if you have a steady hand, you can also position the sticker directly without using water.