I've been meaning to do this for a while. The best way to learn from this is to make a crosshatch strip yourself. I've drawn four so you can see each step, but all you have to draw is one strip and add to it as you go along.

Step 1. Draw an empty strip and fill it with a single tone. One tone is a pattern of parallel, evenly-spaced lines. It doesn't have to be parallel to the area you're filling.

The best crosshatching is done very precisely-- draw each line as close to parallel with the last one as you possibly can. Also don't vary the separation between the lines. These two points are very important but may take some time and practice before you can do it well.

It's tempting to get sloppy, because it can be tedious work, but stick with it. When you get better at it, you'll be able to do it much faster and still have good control and precision. Find a speed of pen movement that enables you to draw the straightest line. Some people move the pen slowly because if they move quickly they warp the lines; I personally find that if I draw too slowly, the lines will look wobbly. Find your balance.

Step 2. Choose a second tone. The spacing between the lines should be the same as for the first tone, but the direction is different. It's better if the direction is more or less random; don't make it perpendicular to the first tone. Overlay the second tone over part of the area filled by the first.

In mine, I have the second tone filling three quarters of the strip, and you can still see the first tone in the bottom quarter. This gives the impression that the top three quarters are darker than the bottom quarter. This is the essence of crosshatching-- darkness and light are created by layering tones that are individually not any darker or lighter than each other. Darker areas have more tones overlapped in one place.

Note at the bottom of the page below the strip I've drawn each individual tone so you can see what they look like when they're not overlapped.

Step 3. The third tone takes up half the strip, again leaving other areas uncovered and therefore lighter.

One important point, especially when the areas you're shading are geometric and have distinct edges, is to know where your edges should be before you start shading, and don't make your lines go outside their designated areas. Note that I had some trouble making the bottom edge of this tone area look straight. I could have been a bit more careful in drawing those lines slowly and ending them at the proper place.

Step 4. The fourth tone takes up the top quarter of the strip.

Now you can see that the top of the strip looks the darkest and the bottom looks the lightest. There is a gradient from dark to light in strips, but it was created by layering equally dark tones over each other. The darker areas look darker because there is a greater density of ink there due to the many overlapping tones.

Conclusion. When you apply this technique to a non-geometric object, you may actually find it easier to decide where the boundaries go between areas of different density. Depending on how light or dark an object you want to draw, you can shade lots of its area or only a little. Decide before you start shading how much of it is going to be dark and how much will be light, and start by shading the entire dark area with your first tone. Then add the other tones over it. It's very difficult to go back and add earlier tones as underlay.

If you're shading an object that's not flat, for example clothing with folds or wrinkles, don't worry about the orientation of the lines. Fill the entire area with parallel lines, disregarding interior borders and edges. When you start adding the later layers, you will make the darker areas butt up against the interior edges so that the edges become defined by their tone.