If you have tried to do decoupage using paper napkin/serviette, you might have encountered this problem when trying to seal or glue the motif onto the surface. As the most outer layer of the paper napkin is very thin, and you use a brush to glue it, the motif will get crinkled easily. From my experience, the bigger the motif, the harder it gets to NOT get it crinkled..isk!
The more you try not to get it wrinkled (as you might use your fingers to adjust the position of the motif) the worse it could get. The design might get torn or damaged in the process.
Try using a bigger size of brush to seal the paper napkin. Load each side of the brush with the sealer or glue, and gently apply the glue on top of the napkin motif, covering as much surface as possible. The best way to ensure a long lasting effect of your craft is to let it air dry for few days (or even weeks if the project is big) before you proceed with finishing process (applying glaze medium and varnish).