Either seek local professional guidance or start reading ... https://www.irs.gov/instructions/i706
Don't forget to check your state's regulations.
Also if the estate has more than $600 of income then a form 1041 will also need to be filed.
When To File
You must file Form 706 to report estate and/or GST tax within 9 months after the date of the decedent's death. If you are unable to file Form 706 by the due date, you may receive an extension of time to file. Use Form 4768, Application for Extension of Time To File a Return and/or Pay U.S. Estate (and Generation-Skipping Transfer) Taxes, to apply for an automatic 6-month extension of time to file.
Portability election.
An executor can only elect to transfer the DSUE amount to the surviving spouse if the Form 706 is filed timely; that is, within 9 months of the decedent's date of death or, if you have received an extension of time to file, before the 6-month extension period ends.
Extension to elect portability.
Executors who did not have a filing requirement under section 6018(a) but failed to timely file Form 706 to make the portability election may be eligible for an extension under Rev. Proc. 2017-34, 2017-26 I.R.B. 1282. Executors filing to elect portability may now file Form 706 on or before the second anniversary of the decedent’s death.
An executor wishing to elect portability under this extension must state at the top of the Form 706 being filed that the return is "Filed Pursuant to Rev. Proc. 2017-34 to Elect Portability under 2010(c)(5)(A)." For more information on this extension, see Rev. Proc. 2017-34.
Note.
Any estate that is filing an estate tax return only to elect portability and did not file timely or within the extension provided in Rev. Proc. 2017-34 may seek relief under Regulations section 301.9100-3 to make the portability election.
