If you are a U.S. citizen or resident alien living and working (or on military duty) outside the U.S. and Puerto Rico, file your 2016 income tax return (Form 1040) and pay any tax due. If you want a 4-month extension of time to file your return, use Form 4868 to extend your filing deadline to October 16. (Note that there are special provisions for military personnel who are stationed in a combat zone. For more information, see IRS Publication 3.)
If you are not paying your 2017 income tax through withholding (or you will not pay enough tax during the year that way), pay the second installment of your 2017 estimated tax. Use Form 1040-ES (Estimated Tax for Individuals). For more information, see IRS Publication 505 (Tax Withholding and Estimated Tax).
Deposit the second installment payment for 2017 estimated income tax. Form 1120-W (Estimated Tax for Corporations) is a worksheet that can be used to estimate your tax for the year.