Pour the cup of tap water into a bowl or cup that you can easily draw from with the eye dropper.
Put black ink into an ink cap until it is 3/4 full. Then add water to the cap using the eyedropper and fill it completely. By adding water, you have diluted the ink. It will turn from black to gray.
Repeat the process of adding water to ink in an ink cap but change the amount of dilution. For example, try using 1/4 pigment and 3/4 water; or half pigment and half water. You will find that one shade or another is more suitable to a particular piece.