For me, one of the most important steps in applying decals is ensuring you have a good paint base, and a good coat of a clear gloss. Decals will adhere better to a gloss finish, as it is smoother then a flat finish, and can really help in preventing the decal from "silvering". "Silvering" is referred to as the excess decal film being seen around the decal. With the decal applied applied correctly and on a gloss finish, this "silvering" can be reduced, if not complete eliminated.
So the first step is getting a good gloss clear coat on the model first and letting it dry thoroughly. For this I use Future. Future is a liquid floor wax, that is sold in stores now as Pledge with Future. This can be airbrushed on and produces a great glossy finish to adhere decals. Remember to clean your airbrush thoroughly when done.
You can also use gloss coats from a spray can as well, for example Tamiya Gloss Coat or Testor Gloss.
Once the gloss coat is fully dry, you are ready to get the decals on. Make sure you have everything you need on hand to start, as you don't want to be looking for stuff while decals are soaking or wet. Some of the tools you will need are a small tub of water, a cotton swap, tweezers, a sharp hobby knife, a piece of paper towel, and decal setting solutions.
You will need to cut each decal from the sheet, as you should only work with one decal at a time, and a new sharp hobby knife is best for this, as it will prevent any damage to the decal sheet and/or decal carrier film.
Now you will need to prep the area to accept the decal, for this I use MicroSet, with a small brush I coat the area where I will be applying the decal.
Then immediately, holding the decal paper with the tweezers, I use a cotton swab to slide the decal into place. You can use a hobby knife for this, but you need to be VERY careful. Also with the cotton swab, you can move the decal around till you are happy with the location.
The cotton swap will also help in absorbing some of the excess water and Micro Set. Once you are happy with the location, use the cotton swap to push the decal down into place, removing any visible air bubbles. Once this is complete, I next apply a setting solution. For decals that are sitting on a flat surface I use a milder solution, like Tamiya MarkFit.
The challenge comes when have to place a decal over raised detail. This is not as hard as it sounds. Apply the decal the same as above, taking a extra care pushing it into place. Once happy with location, you can use a stronger setting solution, I like Walthers SolvaSet, or you can use MicroSol. Apply a liberal amount of the solution on the decal, absorbing any that may be around the decal. Now walk away, do not touch the decal or attempt to move it.
After about an hour, the look my scare you, it might look a little shriveled and uneven, don't worry. Check to see how the decal is setting over the detail, in some cases you may need to do some fine cutting with a VERY sharp hobby knife.
Also if you find any air bubbles, just give them a gentle poke with the tip of the knife. Now apply another coat of the setting solution and walk away again. You may have to repeat this a few times, but it will eventually sit.
And presto, decals are applied, and will have that painted on look. I suggest giving the decals a 48 hour period to thoroughly dry, then apply another coat of gloss to seal the decals onto the model before moving on to weathering.