The sentence "I am sure that you have done the mischief" is a complex sentence, consisting of two clauses. To turn it into a compound sentence, you would need to add another independent clause that expresses a related idea.
For example, you could add the clause "I forgive you anyway" to create the compound sentence:
"I am sure that you have done the mischief, but I forgive you anyway."
In this sentence, the conjunction "but" joins the two independent clauses, indicating that they are separate but related ideas.