Form 2848 is the IRS form for a power of attorney. As you complete your grandmother's tax return in TurboTax, you should complete and print a Form 2848; it doesn't not need to be signed by your grandmother, but your uncle should sign where indicated as the attorney-in-fact, and attach a copy of his general power of attorney document that your grandmother had previously executed. These documents should then be mailed or faxed to the IRS Service Center for your part of the country, based on the "Where to File" chart found here: https://www.irs.gov/pub/irs-pdf/i2848.pdf.
You should then complete your grandmother's tax return, print it, have your uncle sign it, attach another copy of the Form 2848 and power of attorney forms, and mail it to the address in the filing instructions.
