You fill out a W-9. As a US citizen, you are taxed on your worldwide income. It doesn't matter if you don't even set foot in the US.You will however receive a foreign tax credit on your US return equal to the tax paid in Canada or the US tax on the same income, whichever is lower.You also must file an FBAR each year with the US Treasury if you have non-US financial accounts totalling $10K or more. This is measured by finding the highest balance at any time of year for each account and adding up those numbers. Failure to file carries significant penalties.