i've found the easiest way of doing it, was to start with spotless plastics, a hairdrier, and three hands...
start by heating both the plastic bit in question, and the sticker with a hairdrier...
peel off one corner of the graphics/sticker...
make sure it's all lined up properly, then attach the initial point on a corner somewhere...
using the hairdrier, keep the heat on the sticker, and bit by bit, peel the backing off while applying the sticker to the plastic...
the heat will make it much easier to mould and stretch the sticker over bends, folds, corners and curves of the plastic.
when you've stuffed up the first one and discovered how to do it properly, pull out the back up set you bought, and try again.