Hey Rocky:
HOW: As mentioned. The 'ink' does require some form of carrier. I know some use the carrier from Humbrol paints although not sure if this works as well since they changed their formula a couple of years ago. I prefer using either W&N Liquin or Grumbacher Cobalt Drier.
WHEN: Whenever you want. The most obvious is on metal surfaces (buttons, blades, etc). You can vary the tones by adding small amounts of appropriate oils (browns with gold, black with silvers). You can also use them by mixing small amounts of inks into oils for use on braiding (epaulettes, etc), although the trend now is to paint these areas first and then use control glazes of inks as the finishing touch.
I try to paint solid metallic areas first as best as possible. Helps avoid contaminating adjacent areas. Otherwise, smaller areas such as buttons/buckles, etc are left to last. All detemined in the planning stage.
Paul