Complex sentence: Miller spent his early thirties working on staff at Denver's "Rocky Mountain News" where he created "Barney Baxter" in 1935 for the paper's "Junior Aviator" page.
Split into simpler sentences: Miller spent his early thirties working on staff at Denver's "Rocky Mountain News". There, he created "Barney Baxter" in 1935 for the paper's "Junior Aviator" page.