I found this super informative - it uses hatching to build values and form distinction, and the simplified thumbnails really make it easy to follow what's going in and how it works

After editing 'stamp' also known as brush shape or head, make a selection, at the bottom (depending on your layout) selection options, tap 'brush' to add as a brush. In brush settings under the 'head' section activate invert and color buttons.

I made this with my finger on my phone because I forgot a stylus. Ignore the sloppy 'trimming'. It is purely to illustrate that it can be done on mobile.

(Thumbnail from YouTube video)

Uncomfortable(from description): "Some people come out of the womb swinging, brave and ready to take on the world. The rest of us... we're a little more timid, held back by fear and anxiety. In this video, I talk about how I spent many of my younger years terrified and intimidated by the idea of a blank page, and how I ultimately worked to overcome it."