Step 1: Familiarize yourself paths in GIMP. They make a marvelous way to get really precise shapes and stuff, even without a tablet. If you know how to use Inkscape, or some other vector graphics program, you know how to use an even more versatile tool for the same basic thing. General tips:
Use as few nodes as possible for a given thing
Use as few handles as possible for a given segment
Make use of straight lines, the rotation tool, and the basic shapes GIMP gives you access to in order to keep track of geometry
Copy-paste frequently. Whenever you're going to use a shape more than once, really. Paths scale to arbitrary resolutions, so that also helps with reuse.
Step 2: Come up with a design. This is probably the hard part, and it's entirely manual.
Step 3: Assuming GIMP, you'll want to go to Edit>Stroke Path and use that to trace your design, once you've laid down the paths describing it. I've found that the Ink tool can offer some neat effects here, making it look more like a brush stroke.
Step 4: Special effects. In my case, I copy the design, blur it, and then Colorize it to give it a color I want. Set it to Hard Light blending mode. That makes the glow. Then I copy that layer, render some solid noise clouds, Colorize them to a different color for highlights, and paste the glow layer as a layer mask for the clouds. Usually twice, since one paste is too faint. Playing around with brightness/contrast, blend modes, and occasionally filters, is fun here.