The technic of Decoupage and working with napkins is actually the same.
It differs only when the sickness of decoupage - paper that is used is thicker then 40 gr.
The thin decoupage paper (rice paper, silk paper, mulberry paper), has to be used as the napkins so light colour has to be applied before the decoupage paper/s can be glued on the object.
With the 'normal', mostly used decoupage paper (thickness 80 g) it is not necessary to colour the object with white colour before appling the paper. The object can be painted directly with the background color / s and the decopage paper can be applied onto these because the colour will not show through the decoupage paper/s.
The surface must be clean and grease-free like you do with all paper crafts.
Rice Paper Decoupage: prime object and allow to dry.
Decoupage paper (80 g): Colur the object with background color / s and let dry.
Spray water (Not necessarily) on decoupage paper, glue on the paper and stick to subject
Apply on the subject, a thin layer of glue and let is dry.
For decoupage paper (80 gr paper thickness)apply varnish as often as necessary – that you can feel the diference between paper and object.
Between the varnishing processes allow overnight drying and sand down any bumps/bubbles with very fine sand paper and clean the object with a cloth.
Decoupage behind glass
Spray water on the decoupage paper & apply glue on the moitf of the decoupage paper. Stick it behind the glass and apply glue to the decoupage paper.
Once the glue is completely dry, you may remove excess glue using a cotton swab dipped in water or finger nail polish remover. For excess glue along the edges of pictures, carefully scrape it off with a toothpick.
Paint over with glue and onto the rest of the glass from the back and let it dry.
Cover all with primer to get rid of the transparency.
Cover with glue or varnish to get it sealed.