I use clear decal paper. Make sure you get ink-jet paper, since you’ve got an inkjet printer.
I have a jar of Micro-Sol, a jar of Micro-Set and a jar of Liquid Decal Film, all from Microscale.
I use a small paintbrush, a pair of scissors and sometimes a pair of tweezers. All of these are generic and go back into the tool box when I’m done decalling. No “reserved for decalling” equipment here. I blot up excess water with a paper towel.
Computer printers don’t do white. For trains, so far, I’ve planned things so that I print dark-colored decals and apply them to light-colored trains. For structures, I paint a white rectangle (for a rectangular sign) on the building and apply the decal over that, giving me the look I want.
For a clean, smooth decal application, you need a clean, smooth surface. I use a gloss or satin paint finish (or overcoat) to get this. But, if you want a rough, weathered look, then use Dul-Cote, or apply the decal to a flat finished surface like cheap acrylic paint.
The Liquid Decal Film is the coating over the printed surface. You can get spray stuff, too, but I suspect that’s a more expensive option, particularly when you’re doing just a few small decals. And speaking of that, I print 1 row of decals at the top of the page, and then cut them off with a paper cutter so I get another clean edge, and I can feed the same sheet of paper through again. I also set the margins so that the decals start as close to the top of the page as possible. Yes, I’m Scottish.
Once the decal is applied, smoothed and dried, I give it a hit of either Dul-Cote or a clear satin finish.