Buy an extender base that best meets your specific needs. Various art supply manufacturers offer specialty extenders designed to maximize certain properties of the ink, such as viscosity or depth of tone.
Spread a layer of your chosen extender base across a palette.
Add etching ink to the extender base and mix with a brush until well-blended. The quantity of etching ink you add will depend on the desired tone and level of obscurity. As you add more ink, the mixture will become deeper in tone and more obscure.
Ink your plates with the extender-ink mixture and press them as you ordinarily would. Note that extended ink typically takes longer to dry.