This is a video by Nargis Khan, explaining you the techniques of decoupage on scented candle using decoupage napkin and Hair dryer.
This is a very popular technique to do decoupage on a wax surface.
Best tips: Use an oven Glove to protect your hand from direct heat. It gets very hot :)
1) Decoupage Napkin : buy it here : Decoupage Napkin
2) Mod Podge Matte : buy it here : Mod Podge 8 0z
Watch the video in the below link.