I've been cutting (vinyl plotter) and applying decals for about 10 years now. If you have a large decal on a smooth surface make sure that it is 100% clean and clear of dust or dirt particles (leaves air bubbles in your decal). You can use an application fluid on some decals (depends on the media used) that way you can re-position it before locking it down. Stuff I use is bought by the gallon.....which is way overkill for what you want to do so I would suggest trying a drop of dish soap into a quart of water in a spray bottle.....poorboy application fluid. Don't use windex to clean the surface as amonia can cause problems with some printed decals.....alcohol is ok to use to get a clean surface.
If the decal isn't large as in 15" x 20" but is more than you can handle putting down in one shot in fear of wrinkles or air bubbles you can always use the hinge method to apply your decal. The hinge method is to flip the decal over so the backing paper is up and lightly score the backer with a razorblade. That way you can peel only half the decal before the install.....on larger decals a piece of wide masking tape or application tape can be your friend as well. Align the decal put a piece of tape across the top.....lift the decal and peel the top half of the backer and flip that down. Work the bubbles and wrinkles out then lift the bottom half and peel the backer and finish the install.
Here's a youtube video that explains it a little better:
Sorry for the long post but this is something that I have learned on my own over the last 10 years and everytime it gets easier to do an install.
Dragon Slayer 129