Place the vinyl stickers against the wall and mark the dimensions of the stickers with a piece of chalk or pencil. Set them down.
Attach a piece of 80-grit sandpaper to the bottom of an electric sander. Most modern sanders have a rough surface on the bottom that attaches to the soft backing on the sandpaper, requiring that you simply press the sandpaper against the sander, as it bonds like hook and loop tape.
Run the sander over the area of the wall where you plan to place the stickers. Repeat with 120-grit sandpaper, then 220-grit, then 360-grit. Wipe the area down with a tack cloth to remove all the dust from sanding.
Turn the stickers over and dab a few drops of decoupage glue onto the bottom side. Press the stickers against the wall, holding them in place for at least five to 10 minutes so the glue has time to adhere firmly to the newly smoothed surface.