Complex sentence: The WAMPAS Baby Stars was a promotional campaign sponsored by the Western Association of Motion Picture Advertisers in the United States which honored thirteen young women each year who they believed to be on the threshold of movie stardom.
Split into simpler sentences: The WAMPAS Baby Stars was a promotional campaign sponsored by the Western Association of Motion Picture Advertisers in the United States. WAMPAS honored thirteen young women each year whom they believed to be on the threshold of movie stardom.