Cross-hatching is an effective way to suggest tone using line. We can also do this with hatching, without crossing lines, and still communicate plane changes effectively but we'll leave that for another tutorial.
There are a few ways to do cross-hatching but let's go through some of the basics.
We need to identify the light source so we know where to apply shading lines. The lines should go thick to thin, thickest in the darkest parts of the form, thinner as they reach towards the light source. Lines should generally follow the contours of the form though it's not always required, as is the case for large dark areas, for example. We should try to keep lines evenly spaced and parallel or with a gradual angle change if there are subtle plane changes.
It's worth experimenting and developing your own cross-hatching technique. Go very slow at first and build your control.
Effective cross-hatching requires developed skill in line control but practising cross-hatching is a great way to acquire those skills and will enhance your general drawing technique.