In the June 2013 issue of Aeroplane magazine Ian Bott illustrated the article I wrote on the 'Jack' Savage Skywriting S.E.5a aircraft and their pilots. It was a very interesting article to research, and had all sorts of bonuses (it's rare you get a Groucho Marx quote for a start!).
A Savage S.E.5a drawing smoke. [Author's Collection]
Sadly we weren't able to find any contemporary film of the Savage Skywriters in action in the usual places, but we did find an eight-minute 1935 US film: Chevrolet’s "Sky Billboards"
It's American, and a few years later, but explains the task pretty well. (Although the snap rolls are for the camera, not the job.)
The Savage S.E.5a featured a split rudder and joined exhaust-pipes, originally (but not now) wrapped in asbestos, as seen here on the Science Museum, Kensignton's G-EBIB. [J Kightly]
Some other interesting links; Popular Mechanics February 1923, Popular Mechanics April 1925, Popular Science March 1929, LIFE 19 Aug 1940, and also a link to a Corbis image of Cyril Turner, regarded as the doyen of skywriters, here. There is also a August 28, 1937 feature in the New Yorker (available to subscribers only).
A modern Skywriter's account of how the job works can be read online here, and shows, while still a skilled, demanding task, how little it has changed since Jack Savage developed the technique in the 1920s, nearly a century ago.