Bob, there are a number of ways to approach it, some of which will change the tonal quality of the bass slightly (which you might want, who knows?)
The first method, which is best on white skins, is to use a Sharpie or similar permanent marker to draw and colour the design. This doesn't affect the tone any, and is fairly inexpensive. Equally, you can airbrush or paint onto them with acrylic. Stencilling, of course, works just fine.
For all-black skins, if you don't want them custom made and prefer a matte finish, you can dye a white skin using leather dye and a brush or sponge, then apply the design in light acrylic with an airbrush or paintbrush. Equally, you can draw the design on with metallic gel pens if you want very fine, unbroken lines of gold or silver. The dye technique works well for other main background colours as well, and leather dye is available in a wide range of colours.
If you are very adventurous, you can also experiment with finishes that change the tone of the bass - I did a set of heavy heads in flexible heavy acrylic molding paste using a painter's technique called
impasto - this deepened and slightly rounded the tone of the drum. You can also apply layers of cut vinyl (again, done heavily this will change the tone by flattening it a bit) or layers of paper and glaze.
Finally, you can batik some types of head (I'm thinking, primarily, of Remo Batterkings and Ambassadors), using either hot wax or gutta percha as a resist and painting with leather dyes. I prefer metallic gutta (gold or silver is available) for designs dominated by dark colours, and a mixture of beeswax and parrafin (50-50) for designs dominated by light colours, or where I want very heavy lines. I did a set of custom heads for my Banjo in this style.