I was scanning pages out of my notebook (most are non-RPG related doodlings, so they won’t make it up here), and came across a page of interest to those who ask about the crosshatching I do on my maps.
So I’ve blown up the cross-hatching examples specifically for this post, and I’ll try to explain my basic processes along the way here.
The basic start of any simple crosshatching is three parallel lines. Just throw them down somewhere close to the walls you are filling against, but not right on it (yet – once you get a handle on this, I find that it becomes easiest to start at a corner or intersection of multiple walls).
So what you start with is those three lines over on the far right. I them start laying out other sets of lines intersecting and pushing against those lines, trying to avoid 90 degree intersections between them. Try to make the three lines of different lengths, but with the ends producing a nice angle for the next set of lines to mesh with.
Then go nuts, do the same thing again and again until you get something of a jumble like I hate on the left. When you get a close-up of my hatching you see that it is pretty rough with some overlaps, badly-joined sections, and so on.
Really, to make it work is just about practice. Grab a blank piece of paper and just start hatching at random and do a bunch of it. By the time you’ve done half a page it should be coming together almost naturally.
Good luck and happy hatching!