It starts immediately after the formalities of bowing in, before ukemi, and it is cited as fundamental to judo, something that must be mastered to learn judo.
This is incidental to my private discovery over years that many problems in judo and kata can be addressed by first starting with, literally, first steps. Newbies tend to focus on their hands, and get their steps all wrong. I practice with a number of senior Japanese judo sensei - 7, 8 dans - and they, too, tend to teach focusing on hands and gross body movements instead of starting with the steps. But after watching me deconstruct techniques and kata movements starting with taisabki, and often resolving problems that their instruction often did not address, I think I've made a couple of converts.
Taisabaki is very basic to my primary martial art, Nihon den Jujutsu Nihon Jujutsu homepage , and its later cousin, Shodokan Aikido (AKA 'Tomiki aikido') http://shodokanaikido.com/en/ , and practice always starts with taisabaki drills (pretty simple in NJJ, very complex in Shodokan Aikido) and integrated hand movements, releases, strikes, kicks, etc. In fact, the series in "A Detailed Exposition of Judo" seems very close to a Tomiki sensei exposition and some of his prewar writings. So, it's sort of second nature to me, but doesn't seem to be taught as a basic in most judo classes I see. In fact, I've said for some time if I had full control of a dojo curricula, I would start with walking lessons, even before ukemi.
My question: does anyone teach taisabaki fundamentals as part of an integrated judo introductory level course? If so, how? Do you you materials you could share?