The correct answer could be: Charles wanted to buy a new guitar after he saved his money for three months.
Subordinate conjunctions are used to connect two sentences or parts of sentences. If you use a subordinate conjunction, then one of the sentences will be subordinate/dependent; if you use coordinate conjunction, then both sentences will be independent.
In the example above, I used the subordinate conjunction AFTER, which means that the clause he saved his money for three months is dependent and cannot stand on its own.