Here is a little process for one of the bestiary images for River of Heaven (available now on DriveThruRPG).
The brief was for a Raptor alien species. Essentially a mix of dinosaur and xenomorph: savage predators living in hive-like catacomb spires. I wanted a big lot of teeth and ripped a little more from a T-Rex than a Velociraptor just to make it a little different. Long arms with prehensile hands convey a hint of 'civilisation'. I chose purple for the base to, again, make it a little different. Complimentary shades of green and grey were used for details.
Flats. Shading (on a solid layer of black, details erased out and opacity dropped) and Tones (you can see the palette I used at the bottom: colour-picked from here as I went on). Textures were painted onto the skin with rough brushes and picked out of the shading.
All work done in Sketchbook on Samsung Galaxy Note 3 phone.