Here's what my rough sketching is usually like

And here's some in progress screenshots of how I made one of my covers. You can see a bit of the rough sketch under it, but I'd already started the linework.

Linework was almost done here.

Then I did some flats, gradients, and inking. I'd already started a bit of the rendering on the skull.

Last is lighting, effects, and a simple background

When I do digital paintings I just block out shapes with colors and then add details. It's easier to fix things when I'm not using linework so I'll usually use one or two layers. Maybe more if I'm doing a foreground and background.
Here I had a layer with the color palette and one where I did the actual painting. (the other one was a sketch I ended up scrapping, lol)

