Complex sentence: There were always writers who contributed to "Dennis the Menace", these were captions sent to him in the mail by people from around the country, and he would go through them and find one that he liked and draw a comic to match it.
Split into simpler sentences: There were always writers who contributed to "Dennis the Menace". People from around the country sent captions to him in the mail and he would go through them and find one that he liked and draw a comic to match it.