Good advice, Cappy.
I'll add that you need to BE PATIENT with this process. You won't finish it in a day. The gloss finish that you apply the decals to must be well dry -- give it at least a day after you apply the last coat. Also, give the decals a day to dry after applying them before you put any other finishes over them.
The reason for applying decals over a gloss finish:
A property of surfaces called SPECULARITY refers to how smooth or rough the finish is under microscopic conditions affecting the surface reflections of light, but not color. A surface with a high specular nature will be very shiny and smooth. A low specularity will be dull to the eye and, under a microscope, very ROUGH. This roughness of a dull finish will allow AIR to become trapped under the skin of the decal. As the decal dries, it is actually sitting only on the topmost surface of that flat finish. Light is now reflecting off the decal, some passes THROUGH the decal, and reflects on the backside of the decal. Result... SILVERING. By applying the decal to a high gloss, i.e. high specular finish, you give it a smooth surface to evenly adhere to. Light is no longer reflecting through the back of the decal.
Cappy D gave some more good advice: decal setting and solvent solutions are the tools of the serious modeler. What they do is attack the laquer film on which the decal is printed, causing it to soften and settle into the surface over which the decal is applied. As you can see from the issue of gloss vs. flat finish, the better the decal conforms to the surface, in both visible and microscopic detail, the more the decal will look PAINTED ON instead of just hastily stuck in place.
A phenomenon that many people notice is that when you apply a gloss finish, your paint colors will tend to slightly DARKEN. The reason, again, is specularity. That high gloss finish will reflect a percentage of light before it reaches the pigments of the paint itself. Less light reaching the pigment, less light is reflected as color. Think of it this way: you look in a mirror... what color is the mirror? Since essentially ALL light is reflected on the SURFACE, no light will reach the backing to reflect any color of the mirror itself.
Now you can go and tell your doubting friends of the highly technical and scientific nature of model building.