Determine the Change in Circumstances
To modify any family law order you must be able to show a change in circumstances that is relevant to the request. The loss of a job, decrease in income, increase in the other party's income, a new job for the other party, etc. are changes of circumstances that warrant a modification of the child support order. A parent keeping a child from the other parent is NOT a change in circumstances that is relevant to the request to modify child support (although in such cases the "out" parent may wish to pursue modifying custody).
Consider Hiring an Attorney
When you are dealing with financial issues (especially in this financial climate) hiring an attorney seems expensive, but more often than not the benefits outweigh the risks. This is what we do day in and day out, and we know how to present and argue your case in the best light. Think twice (and then a third time) about going without an attorney if the other party is: 1) self-employed, 2) dealing in a cash business, 3) potentially hiding assets/money, 4) the beneficiary of a trust, or 5) represented by an attorney. In some cases, you may actually come out behind in the deal (either owe more support or receive less). Attorneys can help you see this. They know how to run the numbers to determine what the judge will see based on the paperwork submitted. They know how to find and make arguments to show the judge that you should be getting what you request. In some cases the other party could be made to pay your attorney fees.
The Paperwork for an OSC
You will need to file an OSC (Order to Show Cause). The forms are available on the California Judicial Council website (link included) and at courthouses. The typical forms used are the FL-300 (Order to Show Cause), the FL-310 (Application for Order and Supporting Declaration). The FL-310 requires a declaration by you giving your reasons why the amount of child support should be changed. These should be kept to the changed circumstances and NOT about your or your child's wants/needs ("He wants a new iPod", "she needs dance lessons"). You can write a short declaration on the form or attach a more detailed explanation on another sheet.
Don't Forget the I&E
You will also need to file an Income and Expense Declaration (I&E) -- FL-150 that has the latest information on your recent work and earnings. You MUST include the I&E as this document tells the judge the specifics about your changed financial situation. It is the document the judge will use to determine the new amount of support.
File and then Serve and then File
File the paperwork (FL-300, FL-310 and declaration, FL-150) at the clerk's office with the $40 filing fee. They will let you choose a court date that's four-weeks or more away. Then have another adult serve the other party and file a proof of service (you can use an FL-330 or FL-335 or your county may have its own family law proof of service form that you can use). Then be sure to file the proof of service with the Court.
The OSC Hearing
Be on time for your hearing. Check in with the clerk, then sit down and wait until your case is called. Be prepared at your OSC hearing to tell the judge about the changed circumstances. The judge will question both parties, so go in knowing you will be answering questions. Keep your answers and your other statements centered on the changed circumstances that make a modification of child support necessary.