Seems like a fairly common question, so I thought i'd take the opportunity to answer it with a picture walkthrough.
Start with your Nice clean plastic (this one was new)
Make sure that you have a spray bottle with with warm water and a dribble of fairy liquid, just a dribble mind!
Spray a fine mist on to the plastic
The apply the whole decal to the plastic, dont worry it wont properly stick yet....
Line up key features such as holes or cut aways
Gently push outwards from these key features until most is stuck down losely, you'll probably end up with small sections sticking up, thats normal.
Then slowly work your way from the middle out, with an old card or your thumbs, and push the graphic onto the plastic and the water out. This will help to get rid of any air bubbles.
Finally check it over by holding it up to the light. Looking for any air bubbles that may be there.
If you see any air bubbles, dont worry, just peel up the graphic from the closest corner and work it back down onto the plastic from the air bubble point.
Be patient, its not a difficult thing to get right with patience.