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Overleaf offers beginner and intermediate webinars.
Interactive Online Introduction to Overleaf
Below is a free online LaTeX course by John Lees-Miller, co-founder of Overleaf. This course was developed for the University of Bristol and the slides are open source, so they can be remixed and reused.
Three part video course on features of Overleaf. Part 1 covers basic typsetting; part 2 includes creating documents with figures, tables and bibliographies; part 3 focuses on creating presentations with Beamer, drawing in LaTeX with TikZ, and how to track changes in Overleaf.