Complex sentence: To supplement his income he taught actors how to ride horses and shoot guns, and worked at RCA as a technician, helping build the first kinescopic recorder and stereophonic sound recorder for Warner Brothers.
Split into simpler sentences: To supplement his income, he taught actors how to ride horses and shoot guns, and worked at RCA as a technician. He helped to build the first kinescopic recorder and stereophonic sound recorder for Warner Brothers.