This is not legal advice and should not be construed as such. This is a general answer and can apply to anyone seeking to apply for United States Citizenship. To understand the full implications of your particular situation, please consult with an immigration attorney who will examine your paperwork and give you legal advice.
In order to qualify for a fee waiver, you must prove your income first. USCIS will consider the "totality of circumstances" when deciding the waiver request. Make sure you send in information regarding your age, disability, and household income within the past 180 days. Include copies of your welfare status, any other income you have, household expenses, copy of Medicare/Medicaid, lease/rent, bills, income tax records and any other evidence to prove that you can not afford the filing fee.
Be sure to double-envelope the application. On the inner envelope write "N-400 FEE WAIVER REQUEST." On the outer envelope, use the address listed on the N-400 directions. Some locations of changed, so make sure you have the most current version of the N-400 form.
N-400 Form Link:
See this link for the USCIS Memo on what they consider in waiver requests:
If you have further questions, please consult with an immigration attorney so they can go over your application and supporting documents to ensure that you have all the required documents.