I was playing around on my (at the time, new) iPad mini. I had gotten it so I can have a digital sketchbook and escape the hassle of carrying around a ton of art supplied. I’ve explored all kinds of art apps. My most favorite include the ArtStudio app and Sketchclub app (LOVE the community there!). This was done on the Sketchclub app. I didn’t know any fancy schmancy shortcuts on how to do this. I actually still don’t.
So I just painted this the traditional way: make out the shapes roughly then layer, refine, layer, refine, etc. This was done on a single layer, too, just like with traditional painting.
I did this from memory. No references were used.