Generally, lines will be thicker in shadow and thinner on the side of the light.

Inking can also add texture with hatching techniques.

Here are a few examples of inking: hatching, cross-hatching, tearing, dotting, circular lines and patterns.

Of course, there are hundreds of ways to ink a sketch, and it all depends on the objective of the inking. Each artist develops their own technique, and a discerning eye will be able to spot the different styles: comic strip authors, illustrators, mangaka, etc.

Some artists prefer to do the inking after the sketch so that they have the final line before going on to colouring. The choice is yours. However, you'll have to wait for the ink to dry. If it is not completely dry, there is a risk that the ink will smudge and this could darken your colour.

Various inking tools:

• The nib: depending on its flexibility, you can play with thick and thin lines. India ink, which is very black and matte, delivers excellent results when scanned.
• Pens or markers with calibrated tips will give you finesse or thickness depending on the number on the cap.
• A graphics tablet allows you to readjust your lines indefinitely until you're happy with the result.

Examples of finer strokes with calibrated points or with the brush/nib, which allows you to do full and loose strokes with a single tool: 

A. The inking was done before colouring with a marker.

B. The inking was done after colouring, without any drying time.

C. Application of black liner after drying.