Complex sentences are made of at least one independent clause and one dependent clause.
An independent clause contains one idea, is composed of a subject and a verb and can stand alone. In this case the independent clause is "fix yourself a snack".
A dependent clause is not a complete clause and has to be attached to an independent clause to become complete. In this case the dependent clause is "If you start to feel hungry".
Note that if the clause begins with the dependent clause a coma should follow but if the clause begins with the independent clause there should be no coma separating both clauses.
