Here are the step-by-step instructions for filing an extension (Form 4868). Type "extension, filing, automatic" into the search box and click on the "Jump to" link that appears in the search results, then click "Yes" when asked, "Do you want to apply for an extension?", and "Yes" to the first question, "No" to the second question asked on the next screen and Continue. Since you want to make a small payment with your extension request, select "I will need to pay the IRS". The program will then ask you about tax payments you made in 2018 (this includes any withheld tax or estimated payments you made throughout the year. If you didn't make any, leave it blank), then Continue. On the next screen, enter the amount you are going to pay in the box for Estimated Amount I'll Need to Pay, then enter the same amount in the box for Amount Paid with Extension. You'll then Print Extension and mail your payment with Form 4868.
The correct address to mail your extension is found on page 4 in the Form 4868 instructions at https://www.irs.gov/pub/irs-pdf/f4868.pdf
