The linked #NoBadMaps blog post describes some of this, but ultimately there's very little to it: https://kmalexander.com/2019/02/27/nobadmaps/

> Using my brushes is easy: you load them in Photoshop, create a document, and place what you want where you want it with a few mouse clicks. Point-and-click. There’s very little drawing, no scanning, nothing complicated. In fact using any of my brush sets you can make super cool maps in minutes. That’s intentional.

The best way to get started is to just grab a piece of paper, scribble a weird shape on it, and sketch on it where you think certain features go. Everything after that is an implementation detail that tools like these brushes help to solve.

If you've never made maps in any tool that applies symbols or uses "brushes" like this, play around with something like Inkarnate[1] or Wonderdraft[2] first. It's a web tool for fantasy mapping with a basic free version and similar palette of brush-like features that you can paint onto maps, but strips down a lot of the non-mapping tooling and interface that you'd wade through in something like Photoshop or GIMP.

Inkarnate should help give you a hands-on idea of what's possible, and it might be all you want or need out of the process (in which case its "pro" subscription is about $25/year). If you want more flexibility or power after playing around with it, then it's a matter of learning related features in the tool you have or want to use - looking up tutorials on Photoshop brushes, custom brushes in GIMP, brushes and bundles in Krita, etc.

They all work just differently enough to not have a blanket recommendation. These brush packs are in the somewhat well-supported ABR format, but each tool that supports them also has different features for configuring how they're applied. The packs are also available as piles of PNG images that you can place manually or make into brushes yourself, if you're so motivated.

Once you get a feel for "painting" symbols as brushes, it's mostly up to you how you apply them. You might like to draw the outlines of landmasses and bodies of water on paper, scan them, and then apply the features with the brushes. For instance, when I was starting out with mapping I followed Jonathan Roberts's blog[3], which steps through and explains a lot of his process for different types of maps and mapping features from a very basic level of understanding. Each brush set here also includes an "in use" section that links to other brushes the artist used to apply certain effects, like watercolors and textures, in the sample maps.

Maybe you prefer to draw the landmasses in the same tool that you're using the brushes, or maybe you're more comfortable in a different drawing tool. (Some people even use GIS tools to draw maps in a data format transformable to different projections, but that's not a great starting point.) Maybe you'd even prefer to use a random landmass generator to skip the drawing step entirely.

The brushes ultimately exist to save you the tedium of filling in those spaces with repetitive shapes like mountains, tress, buildings, etc., that are all slightly different each time while also maintaining a consistent aesthetic style across the map.

[1]: https://inkarnate.com/

[2]: https://www.wonderdraft.net/

[3]: http://www.fantasticmaps.com/