How does one learn to sky write? It seems like skywriting would be pretty difficult to learn, since there doesn't seem to be any way to step back and see if you made a mistake. And yet people have been doing it since the early days of flying.

So here is my question: how does one go about learning to sky write, and are there any tools to make it easier (a ground mounted webcam with a panel mounted display was an idea that came to mind, have no idea how feasible that is)?
Try reading this article from businessweek.  It's going to give you an answer, but you're probably not going to like it.  Here a quote: 


  “It requires an incredible level of precision flying,” Oliver says. “There are so many factors. Where’s the sun, which way is the wind blowing, where’s your crowd? You only learn that over time, after doing it enough. And you can’t go practice when skywriting fluid is 10 [dollars] a gallon and gasoline is 7 [dollars] a gallon.”
  It’s also difficult to learn the craft of skywriting when nobody is willing to teach it. Oliver and Asbury-Oliver won’t share any of their secrets. In fact, while Asbury-Oliver was employed by Pepsi, she was contractually forbidden from discussing her skywriting techniques with anybody, including friends, colleagues, and journalists. “In the ’30s and ’40s, there were a lot of skywriters trying to make a living,” she says. “So they shared nothing. They wouldn’t tell each other what kind of fluid they used to make the letters, what kind of exhaust pump they used. Everything was top secret.” While an air of secrecy might’ve made sense a century ago, why do they continue to be clandestine, despite their competition being essentially nonexistent? “We’re old enough and we’ve been around long enough that we’re both traditionalists,” says Oliver.