As with most of my Zion paintings, I began with a small pencil comp study on gridded paper -- discovering the light/dark patterns, selecting the format shape, and determining the compositional relationship.
Work now begins on the pocket panel I've selected for the job -- in this case a 5x5 Gessobord which I've pre-toned with Burnt Sienna (a little darker, and redder, than my usual Yellow Ochre) -- as I quickly draw in the key shapes & confirm the shadows. (Note that I decided to draw the chipmunk slightly larger than in the comp study, and eliminate part of the surrounding sandstone.)
Value contrast is relative: lights will appear lighter as new darks are introduced; and darks will seem to get darker as new areas of light are applied. For this reason (and to allow me the freedom to "discover" new highlights and core shadows to the very end) I continue to hold my lightest & darkest colors in reserve.
Once all of the general shapes have been blocked in and the basic color patterns have been completed I turn my attention to the final details; fine tuning color, unifying form, and carefully selecting & applying the brightest highlights & deepest shadows.