Bill and Iain got the high points.
The first step is to gloss coat the mini.
A gloss oat provides a smoother surface for the decal to adhere to. While not a big issue on the front end, this does provide a smoother decal in teh end and since the surface is smoother - less air is able to get underneath the decaql to start the oxidation process. These helps prevent silvering of the finsihed decal.
Next, trim the decal paper as close to the image as possible and use water to release the film from the backing.
There are several ways to do this:
You can soak the decal in water (just don't soak so long it releases from the paper).
You can fold a paper towel into a 3 x 3 inch sqaure and soak it with water and then place the trimmed decal on the towel (image side up) and allow the water to soak into the backing and release the film.
You can follow Bill method... apply water to decal...
Either way, wet the portion of the mini where you intend to place the decal, with the decal still on the backing paper, place it near the final loacation, and then using a wet (with water) paint brush, slide the decal into place.
The dampened paint brush can be used to smooth the decal and to help it reach its final place.
Blot the excess water (very carefuuly) with a Q-tip, tissue, etc. and allow to dry thoroughly before handling.
[If applying multiple decals to the same mini, this may require an extended timeframe to accomplish properly.
Once all decals are in place, apply another coat of sealer. I use matte sealer, but I know several who prefer gloss. It's you mini - so you decide.
As Bill mentioned, there are several solvents / decal aides that can be used... the best known are Micro-Sol and Micro-Set.
These are nice, they really help the final appearance, but they are not required.
I have decaled many of my Starline 2400 series ships w/o using anything other than time, patience, and water.