Here's a good glimpse into my creative process: I randomly drew a circle the other day, knowing that it'd turn into a character's head. Then it became Lisa Simpson's head. Then I decided to draw her underwater so I could experiment with making her otherwise spiky hair wavy. Then I added Alison and Alex just 'cause I draw these three together so often.
And then when I was coloring it, I decided to add self-ink outlines to the girls' hair, as I so often do for underwater drawings. Lisa's hair in particular is interesting to put a self-ink outline on because there's no visible line where her hair ends and her forehead begins, but I think it looks okay. (On the other hand, Bart's hair would probably look a little strange in this situation. Maybe I should try him next.)