Doing a detailed value sketch is a great way to start a the painting process.  It is the time to can work out any design problems, simplify and study the value patterns.  Here is one of the little paintings that I did from my recent trip to Italy.  The value sketch and watercolor painting were done in my little moleskin watercolor journal:

Value Sketch using a soft pencil

This is a tracing of my value sketch done on tracing paper so that I did not have to redraw it for my watercolor painting

Final Painting in my Moleskin Journal

This is the Photo that Inspired my Sketch and Painting