Crosshatching or hatching are useful ways of shading a drawing in pen, pencil or colored pencils. These shading methods work for any drawing medium that can't smudge or blend, and look good over blending on those that can.
How to Draw Crosshatching and Hatching
Hatching, crosshatching and stippling are all ways to fool the eye into thinking that pure black marks on a white surface are really shades of gray. Hatching is the simplest and easiest. Just draw a series of short parallel lines in a row within the shape you want to shade. To get darker shading, put them closer together. You can create the shape by hatching within a light pencil outline and then erasing the outline.
It doesn't matter which direction your hatching lines run, unless you want to create a specific texture with them. Horizontal lines on the sample make it look solid and flat. Angled lines or vertical lines might have a different feeling. Try it different ways and experiment!
Hatching can be curved to show more roundness in the subject. Also lines can be heavier to make the shading darker. Thick and thin curving lines on this sketch of a screw give a shiny metallic effect as they don't quite meet, implying threads, while narrow horizontal hatching shades half the base. Experiment with a brush pen or felt tip that can do thick and thin lines. It takes practice to vary the pressure exactly and gain control of thick and thin lines, but it's well worth it when you need to show textures in ink!
Now we get to the fun part, crosshatching. This is actually easier than controlling thick and thin lines. Draw a patch of hatching that's long and narrow to create a shading bar. To make it easy, do the first layer at an angle lower left to upper right. You can start in any direction, but I find that easiest and so do many artists.
It doesn't need to be perfect, it's an exercise that you can do over and over as a doodle while waiting for anything. Keep a small sketchbook in your pocket and practice crosshatching whenever you're on hold!
Do another layer of angled hatching over the middle of your shading bar in the opposite direction. This is simple crosshatching.
Go over the middle of it again with short vertical lines, to create another layer of darker crosshatching. When practicing, you can scribble these layers and still get effective shading. Remember, the closer together the lines are, the darker the effect with any hatching!
The fourth layer of crosshatching is usually the last for most pen and ink drawings, from there it's easy to go directly to black. But if you want smoother shading, you can repeat this process again and again at slightly different angles until the darkest parts just have flecks of white in black. I scribbled a fifth narrow band zigzagging close to the first angle to put the lines very close together to demonstrate how to get very dark values within the middle of a shading bar.
You can work from both ends toward the middle, shade more in the middle as I did here, or just do a shading bar that's light at one end and dark at the other. Try crosshatching and hatching to shade geometric forms or anything you drew in pencil.
A shading bar is one of my favorite doodles, because constant practice leads to control of line and smooth easy shading. Try it with different kinds of pens. It can also change and shade colors, mixing ink colors, if you do the layers of crosshatching with different colors of felt tips or gel pens for a prismatic look and a richer look than just drawing in those colors separately. Experiment with colored pencils and pens.
Crosshatch and hatch your doodles whenever you're bored. Keep a small pocket sketchbook like a Moleskine handy for when you're on hold or otherwise waiting, it's a great way to practice art and keep stress down. Practice shading bars until you completely control how accurately you can draw short parallel lines, they make a good doodle. Try hatching with dotted lines if you need very light grays in a drawing you're inking.
Don't try to use hatching and crosshatching to create very light values in pen and ink drawings, use dotted lines to continue it if you need to.