Complex sentence: He guested for Liverpool during the war in the 1939/40 season, when all matches were friendlies because of the Second World War and in 1951 he joined Liverpool as manager, following George Kay's decision to step down through ill-health.
Split into simpler sentences: He guested for Liverpool during the war in the 1939/40 season, when all matches were friendlies because of the Second World War. Following this before Liverpool he joined Liverpool as manager in 1951 as a result George Kay's decision to step down through ill-health.