goto your local graphics shop ask them for some stripe or decal application fluid. if they say use soap and water mix, find another shop. there are specific liquids that dont effect the adhesive backing. put the fluid in a spray bottle and use it liberally.
make sure all old adhesive from prior deals are off, and the panels are dust, dirt and lint free. make sure to get a "bondo" spreader from the auto body store for squeegeeing the fluid and air out of the decals.
i usually spray the decal and the panel lightly, then put the decal in place so it is exactly where i want it. with the fluid, you can pick it up and put it where you want multiple times. if it starts to stick too soon, spray alittle more fluid on it. once it is in place, start from the center and work your way out. the edges and uneven surfaces take alot of pacience and attention. some times you slowly have to work at a stubborn area.
dont rush it, is the best advice i can give you. do everything slow and with purpose. air bubbles and wrinkles ruin the look you are going for. ski