I have seen this lesson on several sites, but the one that inspired me first was from Techy Teacher (formerly the Art Kids). I break my lesson down into two, one hour sessions. I introduce lines, patterns and shapes, basic watercolor techniques, complementary colors and the American artist Laurel Burch. I have had success with this project from ages 5 all the way up to 12.
In our first session, we meet the artist and talk about how she uses patterns to make her art interesting. Using lines and shapes, I teach them two to three different ways to draw cats. I let them practice first on newsprint, and then have them use a pencil to lightly draw onto their watercolor paper (I encourage them to draw "big"). Then I let them "decorate" their cats and background. Lastly, we trace over the pencil with Sharpie and erase all our pencil lines that still show.
In our second session I let them add more pattern and color accents with oil pastels-I try to limit them to three accent colors. At this point I talk about complementary colors and encourage them to pick a "color scheme" before they start, trying to use complements by each other. Lastly, we go over basic watercolor technique and they paint their cats. I love seeing how unique each one turns out---don't they make you smile!