There is no one way to answer this question. There are many different ways to diagram or break down a sentence. One of the weaknesses in the field of linguistic is the variety of ways. So, you should ask your professor what he or she is looking for. With that said, there are a few points that most scholars agree with.
First, the subject and verb of the main sentence are central. So, it is best to locate and start with these. Second, look for a direct and indirect object. If there is one, notate it as such.
Third, now you want to look for dependent clauses and you will certainly have at least one, because it is a complex sentence. You will have perhaps a relative clause, a participle, or perhaps a purpose clause. Once you locate it, ask what is it, what is the antecedent (if it is a relative clause or participle) and show the connection.
Finally, look at the other remaining words. Usually they will be time words, duration words, place words, or words in the genitive. In a short, look at every word and ask how it functions in the sentence. See link for a more detailed description.