Here's a values demonstration that I painted recently for my online art course. The aim was to represent a reference image using just 5 values, white to a very dark grey. Details were to be ignored and the focus placed on the major shapes of value. Getting lost in the details is what often leads us astray when attempting to accurately render an image. If you get the fundamentals right, then the realism of your image will work.
The first step is to draw the outline and the division between the light and shadow values.
A quick bit of shading on the sketch helps me make my decisions about which of the five values I will use where.
Most of the challenge with picking the correct values, is comes down to getting past our own expectations of what they should be, rather than we we actually see. In my online course I help people see through these illusions.