clean and dry the areas first,start at the top and dont remove all of the backing at once,get the large outside area nice before you push it around the corners.On the doors wind the window down and pull the rubber back so you can get the vinyl tucked under nice.
Take you time and do it out of the wind or breeze T:
They are not too bad, you can peel them gently back if you get it wrong, its worth having a hairdryer to hand as they are far more workable when warm, i guess you could use water but you will probably not need to.
Pretty much as Aggie said but what I always do (I fit a lot of these at work) is tear an inch strip of backing off about an inch from the top of the decal, that way you can line exactly where you want it without it sticking at the top, when you're happy with it lining up (top and bottom) just wipe a finger line across the bit with the backing missing, not right to the edge of the backing or you'll find it difficult to peel the rest off, peel the top inch of backing off and work upwards, and then start peeling the rest off and work downwards but not all at once, use a slightly downward arched sawing action with your finger to rub the rest on and it will go on lovely, may need to warm the edges to get them round the boor but it depends on thickness of vinyl and as Aggie says do this bit last.
Just seen the last two post, don't try it wet as you'll just end up in a mess trying to squeeze the water out, there really is no need.
Just make sure your paint is clean and dry, and I mean really clean as a tiny speck of dust will show and trap a bit of air around it which will expand in the sun and look terrible.