Request a copy of your federal student loan lender's application for income based repayment. You can usually find a copy on their website, but you may have to call and ask to have it mailed to you.
Fill out the application to the best of your knowledge. You will have to give your personal information, family size, loan information, and possibly other details. If you have any problems answering questions, call your lender and they will be happy to assist you.
Fill out IRS Form 4506-T. This is a request for a transcript of your previous year's tax return, and it authorizes the government to release this information to your lender. You will NOT be approved if you do not fill this form out.
Include any additional documents that your lender requests. If your income is drastically different than the prior year, you should consider submitting alternative documentation for this year's income. This would include documents not older than 90 days showing your income, including pay stubs, a statement from your employer, bank statements, etc.
If your lender doesn't put your account into forbearance on receipt of your application and you can't make payments, call and ask for temporary forbearance pending the outcome of your application.