you apply glue to your case, make sure that it is painted on thin and even. If you have a thick glob or too thick of an even layer, then your paper is going to absorb too much glue and get wrinkly.
Some tutorials will tell you to apply decoupage glue directly to the back of the paper, but this is only acceptable if you are working with thick cards that need to be molded around corners on your surface. For regular paper, always apply a thin layer to your surface, then apply the paper on top of it.
