I've been doing a few of these lately. Process is really straight-forward. I just do a quick line drawing to get perspective down, then grab a couple photos and distort them so the shapes are arranged in a manner that I can use the textures as "paint strokes." Use the clone tool and an assortment of crunchy, expressionistic brushes to lay in loads of texture and imply shapes. Finish it off with lots of selection tool + gradient or a big fuzzy round brush to imply light sources and such.
2 hours, CS6, Lenovo x230t