Tools needed: credit card or drivers license, windex, and some 400 grit sandpaper
If your plastic is scratched, or has some bumps on it, take the 400 grit sandpaper and smooth the rough parts down. After you get done sanding, wash off the plastic with water, and wipe it dry. Afer its dry peel off the graphic backing. Spray the entire decal backing (sticky side) with your windex. Set the decal on the plastic. Use your credit card to work the windex out. Press hard, and take your time. Some edges will stick up, but keep working the water out, and pressing hard, they will eventually lay flat against the plastic.
Using the windex will allow you to move the graphics around without getting bubbles or misalignment
I found this on another site and just consolidated it on how i did it and it worked great