I personally use Duncan’s Gold Luster Overglazes. They make: “normal” bright gold if you just want to get your feet wet, premium bright gold for higher end work, a nice white gold, and mother of pearl luster.
Each bottle is tiny – only 2 grams. But it will probably go farther than you think. I’ve read that 1 2g bottle will cover a 12” vase entirely… of course, how fat that vase is would be anyone’s guess! If I’m just adding rims to my tiny 2” ring dishs I find that the bottle lasts for a long time. But if I’m covering the entire handle on a set of large mugs, the bottle doesn’t really last for more than 5-10 mugs.
I use a small brush designed for oil paint (I just get them at my local craft shop) to apply the luster. I also make sure that this brush is dedicated just to gold luster, so it doesn’t get contaminated with anything else.
While this guide is specifically for gold luster, I have also worked with Duncan’s Mother of Pearl luster. It is much more forgiving to use, and I fire to the same temperatures as my gold. My main tips on mother of pearl are 1. do NOT paint the gold over mother of pearl, or vice versa, and 2. the brush strokes tend to show up after fired. I personally find the pearl most beautiful when I have painted my strokes all in the same direction.