Complex sentence: Morris taught himself embroidery, working with wool on a frame custom-built from an old example, and once he had mastered the technique that he trained his wife Jane but also her sister Bessie Burden and others to execute designs to his specifications.
Split into simpler sentences: Morris taught himself embroidery, working with wool on a frame custom-built from an old example. Once he had mastered the technique he trained his wife Jane, her sister Bessie Burden and others to execute designs to his specifications.