first step of classical realistism painting

I really enjoy painting this!! Not easy though. It is my first attempt in using this technique.
This is a real Work in Progress. This is the first layer of the underpainting in raw umber and titanium white.
 I am working at the slice of the zitron and starting to paint the dark background.
Untill now, I think I painted for 12 to 14 hours. Long way to go.