Here is the figure of the Virgin:
OK, here is the first stage of the underpainting on the actual panel:
The dark area behind the head of St. John was a place where the gesso was too absorbent and took up too much of the imprimatura.
After I applied the imprimatura, and it dried, I worked up the figures in a grisaille: I used black and white paint to make a complete "grayscale" of the figures. You can see the completed grisaille of St. John on the right of the panel. After all the figures were painted in grisaille, I then began to glaze color over them.
I really should have made more photos as I worked, but I got into the painting and forgot. This weekend I'll photograph the finished painting and show here. I'll also say a lot more about how I worked and other sources for the painting.