Tutorials must be created by you and your own idea.
If you got idea from elsewhere and revised your own version of tutorial, please include reference and credit original authors.
For example if you found a Photoshop tutorial and wanted to write a GIMP version, you must credit the original Photoshop tutorial.
I'm trying to follow the tutorial using a plain black image (a black blob I made with the brush tool) to create a watercolor photo mask similar to what I posted above. In Step 2, the tutorial says to choose several (6-10) areas for the posterizing. I don't understand how to select the various areas. Also, is this step supposed to somehow create a second layer of the image? It didn't on mine (GIMP 2.10.8).
Two different techniques (the one explained in the tut and the one using brushes, mentioned in your previous post).
Step 2 of the tut simply says to convert the image to indexed mode using around 7-8 as number of colours, then bring it back to RGB; it also says not to apply the following steps (gradients) on whith-ish areas, because it will have no effect. So that in some cases you can use 10 as number of colours for the conversion but then operate with the gradients only on 7-8 areas.
Each area can be selected by Select by color tool.