You will be asked to attach the appropriate income documentation for your Income Driven Repayment plan application at the end of your Savi application flow. This means after you enter your personal details (Address, Date of Birth, etc), and once you click "Review Application."
Regardless of how your income will be documented (typically either a paystub or a 1040 tax return), you will attach this documentation directly to your Savi application! After you choose the income documentation type (within Savi) you will be using, you can continue with entering your SSN and signing.
You will then be able to review your form and submit.
You can visit this article for more information on which income documents you can use, as well as troubleshooting tips: https://bysavi.zendesk.com/hc/en-us/articles/24850054983443-What-type-of-income-documentation-should-I-use-on-my-Income-Driven-Repayment-Application