Complex sentence: Beginning with 1936, he was a bona fide director, first doing comedy and musical shorts for RKO, but in 1938 he graduated to directing features, his first being "Rebellious Daughters" for the low-budget Progressive Pictures.
Split into simpler sentences: Beginning with 1936, he was a bona fide director, first doing comedy and musical shorts for RKO. In 1938, he graduated to directing features with his first film being "Rebellious Daughters" for the low-budget Progressive Pictures.