The natural-law argument states that because there are consistent and predictable natural laws in the universe, there must be a law-giver who set those laws in motion. That law-giver is assumed to be God.
This argument relies on equivocation between two meanings of the word "law".
Legislative laws, such as "Do not murder" or "No littering" are prescriptive: they are established to demarcate acceptable and unacceptable behavior. If a person breaks such a law, he or she has committed a crime, and may be subject to punishment.
Natural laws, on the other hand, are descriptive: they describe how some aspect of the universe behaves. For instance, Newton's law of motion "F=ma" describes how solid objects behave when acted upon by a force. If a person or object breaks a physical law, then it is the law that is in error, since it obviously does not adequately describe what it seeks to describe.
Natural laws by convention
Bertrand Russell wrote:
"We now find that a great many things we thought were Natural Laws are really human conventions. You know that even in the remotest depth of stellar space there are still three feet to a yard. That is, no doubt, a very remarkable fact, but you would hardly call it a law of nature."
- — Why I Am Not A Christian