A friend runs a vinyl cutting business, ive done some work for him before applying vinyl to vehicles. We normally use a small plastic paddle type thing, which he calls a "squeegee" (dont know if thats the correct technical term). Depends on the size of your vinyl, I would take the backing off and apply a bit at at time pushing from the middle to the edges quite firmly (to get excess air out), then do another section as you go. If its big be carefull not to stretch the vinyl, but hold it taunt as you do it to avoid crinkles, then pretty much as mhermann said get a pin and poke any airbubbles out. (9/10 theres gonna be airbubbles of some sort I would say). If its a small graphic, just wack it on there as you want it then again push from the middle outwards throughout the graphic and peel off the backing gently, might also be handy to have a scalpal for any lettering (if you have any) that might get crinkled or shifted, you can slice underneath them take them off and reapply if done carefully.