Begin by having your kiddo scribble on a piece of paper with a regular, white crayon. You can use this time to talk about abstract art, and let them know the scribbling doesn't need to be any particular shape or design.
Have your kiddo color over the paper, right over the white crayon. The harder they press, the darker the finished product will be. My daughter said, "light, Mommy" so she kept her coloring light and airy.
Next, have him or her dip a paintbrush into some water and "paint" over the watercolor crayons. My daughter loved how the watercolor crayon colors started to mix and blend!
Keep going until you are happy with how everything has blended. Be careful not to use too much water!