I use distilled water because it doesn''t stain the surface of the
work like common tap water tends to do. I use a small black plastic
tray from some frozen dinner for a bath. The dark color keeps the
lighter decals visible. Before I start, several drops of solvent are
added to the decal bath.

I slide the decal off the paper and onto the model. I drain some
water off with tissue, but try to leave enough water to position
the 'cal. When all is right, I remove the rest of the water; if it
isn't right, add water back to the film and reposition the decal.
Next I introduce setting solvent [I use Champ's] by bleeding it along
the edges of the 'cal with a fine brush. This allows the solvent to
leak under the decal without moving it. After this solvent has spread
under the film, I add more solvent to the top of the film. When the
decal has dried, the model is ready for overspray. I've had great
success with this method on plastic freight cars, diesels and brass.

hoghead32