You can proceed in the same manner by which you obtained the original order. See Family Code 4009: "An original order for child support may be made retroactive to the date of filing the petition, complaint, or other initial pleading. If the parent ordered to pay support was not served with the petition, complaint, or other initial pleading within 90 days after filing and the court finds that the parent was not intentionally evading service, the child support order shall be effective no earlier than the date of service."
You might also consider contacting the Department of Child Support Services to gain their assistance in pursuing this order on your behalf. Alternatively, you might calculate the amount you believe you are entitled to and attempt to work out a reduced settlement with your ex to avoid going to court.
Please be advised that this answer in no way constitutes legal advice, and is only intended to guide you in determining an appropriate direction for individualized legal consultation and/or representation. This answer should not be relied on, as each legal matter, and the appropriate course of action, is entirely dependent on the specific facts of your particular case. You are encouraged to seek the advice and guidance of a respected and experienced attorney practicing in your community to assist you. Please be advised that this answer does not create an attorney-client relationship and these communications are neither privileged nor confidential.