Soak both the surface of the car AND the peeled decal with soapy water. Place the peeled decal on the car, slide it where you want it, eyeball it ten or fifteen times to be sure it's where you want it, then squeegie it out real nice. Make sure it's good and dry, remove the backing, and you are golden.

(Make sure it's really really dry)

Installation video here...

http://www.youtube.com/watch?v=eBGIQ7ZuuiU