Learn the geometry of snake scales to help you sketch in the field.
Don’t drive yourself nuts trying to copy every scale exactly. You can suggest scales with the X technique (demonstrated below), add a few details and you are good.
Draw an x pattern over the back of the snake. Each of the scales will fit into one of the spaces between the lines. The body scales of the Ring-necked Snake below are simply an X hatch overpainted with watercolor and little highlights added on each scale with a white colored pencil. The effect is convincing and fast.
X hatch lines are often at an oblique angle forming small diamonds instead of squares.
X hatch scales turn to interlocking S curves when they foreshorten and wrap around a cylinder. However, many snakes are more triangular than round in cross-section and the scutes or belly scales interrupt the S pattern on the bottom of the body. Still, you often see a subtle deflection of the hatch lines as they approach the back. You can also just stop the X pattern just short of the edge as the foreshortened scales are more difficult to see.
Having realistic expectations of what you can get in the field will help you work more efficiently and be happier with what you do. The snake heads on this page are scientific illustrations. They were drawn over several days with extensive reference material and a comfortable chair. In contrast, the studies below, showing the body colors and patterns from the dorsal line to the scutes, took only minutes to complete and convey lots of information about patterns on the snakes. This is a great approach for field sketching.
My favorite website for reference material is California Herps. I am grateful to Gary  Nafis for letting me use some of his photographs for reference and step-by-step tutorials.