How do you start and proceed a complete syntactic analysis of a complex sentence? The asignment is to create a tree-structure, by dividing the sentence in to phrases and clauses.
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.
The only thing I would add to the first post is a simple reminder for you about the difference between independent and dependent clauses and phrases because the assignment is to divide a complex sentence into clauses and phrases. An independent clause has both a subject and predicate, makes sense by itself, and can stand alone as an independent sentence. Be sure to look at the conjunctions as being either coordinating or subordinating which will help. A dependent clause has a subject and predicate but cannot stand alone as it uses a subordinating conjunction such as "because" to connect with an independent clause to make sense. A phrase contains two or more words to convey a thought in the sentence but uses neither a subject nor predicate. Remember to show the connections as part of your answer.