Complex sentence: He joined the American Enterprise Institute in 1989, where he was the institute's John M. Olin fellow, before later joining the Hoover Institution as its Robert and Karen Rishwain Fellow.
Split into simpler sentences: He joined the American Enterprise Institute in 1989, where he was the institute's John M. Olin fellow. He later joined the Hoover Institution as its Robert and Karen Rishwain Fellow.