This illustration is a workout on the play of lights between different materials.
The first step that I realized is a very fast sketch :

Digital Painting Bocal citrons 01 Lady Melusine
I then put all the main colors :

Digital Painting Bocal citrons 02 Lady Melusine
I then focused on the shadows only :

Digital Painting Jar of lemons 03 Lady Melusine
Then I worked the different lights :

Digital Painting Jar of lemons 04 Lady Melusine
And finally I finished with the cap of the jar :