Set out a number of unused inkwell cups. Fill each cup with a small amount of ink. The more ink you put in the cup, the more difficult it will be to thin it out.
Mix a small amount of distilled water to the first of the cups. The ink will be thinner and lighter. This technique is often used to give a wash look to tattoo shading.
Mix different amounts of distilled water into the inkwell cups so see how thin and light it makes the color. Stir the ink and water well to make sure that they are thoroughly mixed.
Continue mixing -- in different cups, if possible, to keep track of the water-to-ink ratio of each attempt -- until the ink has reached your desired shade.
Create as much of your desired ink mixture as you will need for the specific tattoo.