Preparing Items For Decoupage

Wood
Lightly sand any rough spots. Fill rough spots and holes with wood filler and then sand. Wipe with a damp sponge. If you want a stained background, use a water-soluble, alcohol-based wood stain. You may need to seal the wood.
You can apply your cutout directly onto untreated wood, but it’s better to apply a sealer. Before sanding wood paint a base coat first as the water-based paint lifts the fibers of wood way from the surface so when you sand the paint the fibers are easily removed. The first coat of paint highlights imperfections that you can correct with filler.
When using a wood base, remember that oil and water don’t mix. The most common decoupage glazes are water-based and won’t adhere properly to wood that has been stained with an oil-based stain.
If you’ve stained wood with an oil-based stain, you need to seal it with an oil-based sealer. If you want to decoupage a piece of old furniture and aren’t sure of the finish, find an inconspicuous spot to test your glaze.
Glass
Wash your glass item with soap and water, rinse thoroughly, and wipe dry. Wipe the surface with rubbing alcohol or vinegar and allow to dry before working on the surface. Don’t touch the surface once it has been cleaned as oil from fingers can cause your images to not seal properly.
You need to be extra careful to remove any excess glue from the glass surface, once your prints are applied. It's also very important to work on a clean surface. When using PVA glue on glass, add a drop of glycerin to prolong the drying time. Once you have applied your cutout to the glass surface and allowed the glue to dry, you can remove any glue by wiping the glass with a window cleaner. Don’t apply glue to the image, only to the glass. Don’t worry about leaving too much glue between the glass and the image as it will dry clear. The more glue you work with, the easier it is to get rid of bubbles. Similarly, it’s ok to leave a thin film of glue both on the glass and the back of the image.
Spray a small amount on a damp sponge or cloth and remove any excess glue from the glass. Cotton swabs work well to remove glue from small areas.
Plastic
To decoupage on plastic, you need to roughen the plastic first with sandpaper. This will give it some tooth with which to grab and provide a tight seal. You will need to allow extra drying and extra curing time than with other surfaces. Dry to the touch isn’t enough. Lightly press your finger in an area that won’t show to test it if possible. If not, allow several days for drying to be on the safe side. Keep the object protected against dust while it’s drying.