First draw wavy line vertically, then draw a single line horizontally (see right side of image). Fill the space between vertical lines with either a convex or concave line depending on if it is above or below the horizontal line
I loved doing this in art class. Art With Mr. E: Line Design w/Shading - Grade Very cool, fun art shading project from Art With Mr. he's an elementary art specialist. Try this with your kids (but I think grown ups would enjoy this too).