My ex lives in another state, is "underemployed" and is in the process of being prosecuted for criminal non-support due to back child support over $20,000 dating back several years. I make too much for legal aid and don't make enough to hire an attorney. Can I represent myself? How do I do it?
If he is being prosecuted for criminal non-support, the prosecutor will handle any defenses your ex may raise.
If your ex is also trying to file in the civil court, they cannot retroactively modify support prior to the date of their motion. There may also be an issue of whether or not the court should impute income.
Your getting into some fact-specific areas though, so if you want to represent yourself you'll need to spend time at the law library familiarizing yourself with the law so that you can argue how the law applies to the circumstances of your particular case.