Like other tasks prep is the most important step.
obviously, ensure the surface is very, very, clean.
have some kind of plastic squeegee, a credit card will do in a pinch but far from ideal.
hang the decals on with low tack (blue/green painters) tape, in the position you want them.
have a spray bottle with (distilled preferred) water or a very weak water/soap mix. You will spray this on the car just before you apply the decal. It will give you more "open time" to ensure the decal is on straight and allow you to squeegee out air bubbles.
those are just some of the basics when working with vinyl. others will chime in with more I am certain.
Good luck! Photos of before and after please
