Here's a small spot illustration called "Strategy Session" that I did for the back cover of a science fiction paperback in the late 1980s, showing a group of interplanetary military types planning their next move.
Here's the first concept sketch from imagination, drawn with a pen and markers. I did four or five of these sketches, and the art director and I chose this one with a red dot.
The next step was to work out each of the creatures. I did this charcoal study of "Hammerhead" while wearing an old costume and looking in the mirror. Yeah, that's me posing. That's pretty much how I look when I try to pull an all-nighter.
I like doing studies instead of taking photo reference not because I want to be low-tech and classical, but because this method is more practical. It's faster than taking a photo—or at least it was faster when you had to get photos processed overnight. But more importantly, it gets me thinking about artistic choices right away, and I'm not swayed by incidental details.
Once I had all the studies, I worked them into a line drawing, which I transferred down to the panel in preparation for the oil painting, which is about 6 x 12 inches.
The painting appeared on the back cover of The Fleet #4: Sworn Allies by David Drake