If you do not pay in full when you file, you will receive a bill. This bill begins the collection process, which continues until your account is satisfied or until the IRS may no longer legally collect the tax, for example if the collection period has expired.
The first notice you receive will be a bill explaining the balance due and requiring payment in full. It will include the tax plus penalties and interest added to your unpaid balance from the date the tax was due. You can pay by sending the IRS a check or money order, payable to United States Treasury, with a copy of the notice. You may also pay by credit card; debit card, or Direct Pay from your bank account. Check out Paying Your Taxes on the IRS.gov website for additional information.