In most cases, you can find the necessary forms on your lender's website. If not available, you can call your student loan lender and they will mail you the forms.
Once you have the forms, you will need to mail back a minimal amount of documentation that includes the completed form and an income verification (bank statements, pay stubs, and/or a tax return).
Once these have been received by your lender, information on your new payment plan will be sent to you within 30-60 days.

