Complex sentence: He started practicing by giving catechism classes to young boys in Castiglione in the summers, and by repeatedly visiting the Capuchins and Barnabites in Casale Monferrato, the capital of the Gonzaga-ruled Duchy of Montferrat where the family spent the winter.
Split into simpler sentences: He started practicing by giving catechism classes to young boys in Castiglione in the summers. He also repeatedly visited the houses of the Capuchin friars and the Barnabites located in Casale Monferrato, the capital of the Gonzaga-ruled Duchy of Montferrat where the family spent the winter.