Citadel Nuln Oil Shade is a very thin paint designed to follow and accentuate the details of the miniatures. These types of paints are also called washes. They are invaluable to any painter. Shake Nuln Oil well before use.
Using a brush, remove a small amount of black paint from the jar and onto a palette. Apply the paint to the model with a slightly larger brush, so that a lot of paint can be applied. To do this, place the tip of the brush on the figure where the details can be found and let the black paint run into it. The paint will follow the shape of the details and give a natural "shadow". Play with the amount of Nuln Oil you apply so that it is not too much (or too little). You'll get the hang of it soon enough.
The color of Citadel Nuln Oil (24-14) is similar to: The Army Painter Dark Tone Ink (WP1136); Old Citadel Badab Black Wash; Vallejo Game Color Black Wash (73201); Instar Black; Privateer Press P3 Armor Wash.