1. Do a messy, blobby shape with a light source. Set a horizon. Use a big brush, don't get bogged in details.
2. Refine the shapes, push things into the back by deaturating and lightening. Bring them front by darkening and saturating.
3. Make sure the piece has a background, midground and foreground. Set something recognizeable in there for scale (tree, person, bird etc)
Just keep it loose as long as possible. The happy mistakes and strange shapes you get from that gives stuff that you couldn't have thought of first.
Making your own brushes or using a set of painting specific brushes is key. I basically carve out the shape and add lighting then give the surfaces texture with specific rough edged brushes and it's done.
I'm really looking forward to going further with this, wish I could draw personal stuff at work!