I usually begin with a toned canvas, and measure and place the figures with marks made with a small brush and thinned raw umber or terra verte. Next, spots of color indicating dark/light bookends.
It's a good idea to commit to the background right away, and to start bringing up the skin tones.
The time between images three and four take the longest - lots of fussing with the likeness, edges, and color and value transitions. All fun:)