A simple sentence may have a compound subject or even a compound predicate. Sometimes, it might have both.
Jim ate the peanuts. Simple sentence with one subject, Jim, and a simple predicate, ate.
Jim and Janice ate the peanuts. simple sentence with a compound subject, Jim and Janice
Jim and Janice ate and burped the peanuts. simple sentence with a compound subject and predicate , ate and burped- both subjects doing both actions
To make a compound sentence, take two sentences and join them together with with a comma and an appropriate conjunction OR put a semi-colon between the two sentences.
Jim ate the peanuts, but Janice burped in sympathy for him.
Jim ate the peanuts;Janice burped in sympathy for him.