You can add a clause at the beginning, middle or end of a simple sentence.
E.g. – Strolling out of his house, Reginald realised that his laces were untied.
Reginald, who was strolling out of his house, realised that his laces were untied.
Reginald realised that his laces were untied as he was strolling out of his house.

Change these simple sentences into complex sentences by adding another clause.

The police bought everyone a McBeanold meal.
Vincent smacked Bert around the head.
Bert demanded some large fries.
McBean had to look after the restaurant.
McBean slipped on a banana milkshake.
The detective drove the thieves to the police station.
Reginald returned to the restaurant.