Since the dialect is more or less dying and the available resources and descriptions are aimed at linguists rather than language learners.
The linguist Hans C. Boas (University of Texas at Austin) has been recording the dialect in order to preserve it, which is a very different goal than the creation of learning resources. See the the YouTube video "Speaking Texas German" and Texas German Dialect Project (TGDP). The project's references page lists mostly academic publications by Hans C. Boas and by a number of other researchers, including Glenn G. Gilbert (emeritus professor at the Southern Illinois University) and Fred Eikel. Gilbert and Eikel did a lot of research on Texas German in the 1950s to 1970s. According to the article ‘Der Cowboy’ Riding into the Sunset (Reporting Texas, 07.12.2014) their studies "provided English translations for lists of Texas-German words, phrases and sentences."
Hans C. Boas says in the YouTube video that Texas German is hard to find nowadays; for example at nine-pin bowling alleys, particular types of social clubs such as dancing clubs, shooting clubs, card-playing clubs, specifically in New Braunfels and Fredericksbug. Some of the commenters on that video claim that "Texas German just sounds like normal German!", but the WikiTongues video in the question and the YouTube video A German Reacts To Spoken Texas German From The USA (by Get Germanized, who discusses a few WikiTongues videos) should make clear that there are differences in the grammar and pronunciation (the "southern drawl"), at least when compared to High German. These differences shouldn't cause major comprehension problems, though, so people fluent in German (not only native speakers) don't need to do anything special in order to learn to talk with speakers of Texas German, except for being prepared to insert certain English words or explain certain German words. For example, one of the women in the YouTube video "Day 95 Part 1: Texas German - Guten Morgen Y'All" says that there are words from High German that she doesn't understand.
People who don't speak German need to rely on resources that weren't designed as learning resources; many of those (especially books and articles) are of a linguistic or historical nature. For example:
- Hans C. Boas: The Life and Death of Texas German (Duke University Press, 2008). This book is described as "the first major study of Texas German as spoken in the twenty-first century, focusing on its formation and the linguistic changes it has undergone."
- the articles on the TGDP project's references page;
- the resources in the TGDP's Dialect Archive, which requires registration.
However, due to the nature of these resources, the best way may be to learn Standard High German and then learn the peculiarities of Texas German. This roundabout way is inspired by the fact the German Texans Heritage Society offers online classes using the textbook series Menschen, which is series for Standard German.
(There is also a German book about the language: Deutsch in Texas by Marcus Nicolini (LIT Verlag, 2004); this book focuses on a linguistic description and the history of the German language in Texas.)