How shade a tattoo?

same as drawing when you shade:)

Secondary answer, There are many techniques to tattooing. Shading is mainly based on your artistic style and knowledge base. Pointillism shades by the density of dots, Griding Shades by basically scribbling one direction then the other, IE; vertical then horizontal, then there is the Circular, small circles slightly overlapping for maximum coverage. Line shading, by increasing or reducing the density of the lines. An artist of any worth can apply this and several other techniques to their craft.