Filing Taxes as a freelancer

Hello and wonderful question for tax planning.  In respect to freelance income outside of US you will include on your tax return.  I am assuming US citizen or resident alien, etc. that would qualify for regular 1040 return with a schedule c.  Your freelance income would be reported on a schedule c.  That form allows income and related business expenses to be reported.  You are correct in having to make estimated tax payments every quarter.  Based on your best knowledge you will assess your income for the year quarterly to determine if an estimated payment needs to be done and how much.  I will link a tax estimator and 1040ES forms from IRS website that shows a manual way to complete with due dates or use the link (tax estimtor) to assist in arriving at that calculation.  I hope this answered your question.  

1040ES 

Tax Estimator