Ok, there you sit with a car and a decal...lets get to work...here are the steps.
1. If you are using decals printed on ink jet or laser jet paper you will need to carefully trim the decal. If you are using clear wet slide decals and are applying to a dark surface you need to consider the back ground color before slapping that decal on.
+for instance, you have the word POLICE in white lettering and it is surrounded by black, you want to apply it to the back of a black trunk lid...if it is clear decal paper you must prep the surface first.  Simply apply a smooth coat of white paint upon the black surface, keep the white surface just a little larger than the actual POLICE lettering.  Ink Jet and Laser Jet printers cannot print white ink, therefore when creating the decals you have to trick the computer...the decal as it is printed looks white but that is from the white color of the background paper, when you dip and slide that decal from the backing, the decal will be clear where the word POLICE is. By applying the white paint back ground you have basically added the white that the printer couldnt.  This is good for lighter colored decals like in the POLICE lettering in the photo.
Once the white paint has dried you are ready to apply the decal.  If there is any white paint still showing, dont worry...just border the decal with black paint and there you go.  Black acrylic from Testors works great!.

2. Carefully dip the decal in to room temperature water and remove, I stand my decals on edge and then I go pour a cup of  coffee.  Let the decal absorb the moisture.

3. Insure the surface you are using is clean.

4. Gently apply the decal by sliding it in to place  (Yep, that is why they call em 'wet slide').  Ok now smooth the decal with a damp cotton swab.

5. Allow sufficient time for the decal to dry.

6. Paint a clear coat of clear nail polish over the decal.  Lightly coat the decal.  This will do two important things....seal the decal so they wont peel later and it will prep it in case you need to trim.  As an example...you have a decal that covers two doors of a car that has opening doors.  Great, once the clear coat is dry, use a surgical scapel to slice through the door opening. 

Tools needed:
Soft fine point paint brush.
Scapel...I buy mine by the case here on Ebay...these work much better than hobby knives as they have a much sharper edge.
Cotton Swabs.

White Backed Decal Paper
There are several brands, some white paper doesnt cover dark backgrounds, some do...the gel type white decal paper is best.  CAUTION:  you need to use very sharp scissors for trimming white backed paper.  The clear coat that is applied to the decal after printing can become a little brittle. If you use less than sharp scissors...you could chip the edges as you cut. 
Apply the decal as you would the clear wet slide...you may need to trim paint...have fun, enjoy the hobby.

Jack at AFTERMATH GRAPHICS

This is a great hobby, I wish you the best...if you have questions...feel free to write me at jackbroadhurst@hotmail.com  I work graveyards and try my best to check emails daily...if you dont hear from me right away...just be patient, Ill respond to your inquiry as soon as I can.  Work safe, have fun and enjoy life...positive thinking brings great rewards!