Complex sentence: He supplemented this training with a comprehensive course in stenography, after which in the spring of 1881 he became bookkeeper for Thomas J. Watson, at that time the leading oil broker in Pittsburgh.
Split into simpler sentences: He supplemented this training with a comprehensive course in stenography. After learning stenography, in the spring of 1881 he became bookkeeper for Thomas J. Watson, at that time the leading oil broker in Pittsburgh.