1. Begin with a careful,detailed sketch
Drawing is the first step of a watercolor painting. Begin by sketching from a live subject. You can start off loose or you can start off careful, but ultimately you create a precise line drawing. Use a 2H pencil to transfer the line drawing to140lb watercolor paper. These lines then become "guidelines" for the watercolor painting.
2. Work up a value study
Capturing light on form is essential. Before even picking up a brush, you need to resolve the values, or the range of darks and lights within your drawing or painting. Place a piece of tracing paper over the line drawing that you've done and begin shading with pencil. Make sure there is a consistent, strong light source and really study the light as it falls over the form of your subject. This value study becomes a resource to look back at as you paint your subject.
3. Pale washes are a good way to start
Begin with a layer of pale wash. Some artists lay in a flat wash and others start right in with graded washes. Choose the lightest hue that you might see in a highlight area or a somewhat brighter hue that you might like to see shine through your outer colors. Make a plan for painting--you might paint over the whole subject at once, or you might complete one small area before moving to the next.