Do I report the missed RMD in the year I should have taken it, or the year following when I receive the 1099R

A missed 2017 RMD taken in 2018 is reported on the 2018 tax return along with the 2018 RMD.  Nothing goes on a 2017 tax return because there was no 2016 distribution.

Download the 5329 form from the IRS website and fill it out the same as above.  Print and mail your return with the 5329 form and explanation attached as described in the 5329 instructions.

You will need to file a 5329 form and request a waiver of the penalty.  (The waiver would only be denied if there is no reasonable explanation and the missed RMD was not taken at all.)

It is not necessary to amend 2017 to sent the 5329 unless there is another reason to amend that changes the taxable amount.

From 5329 instructions:

Quote:

"Waiver of tax.   The IRS can waive part or all of this tax if you can show that any shortfall in the amount of distributions was due to reasonable error and you are taking reasonable steps to remedy the shortfall. If you believe you qualify for this relief, attach a statement of explanation and file Form 5329 as follows. 

   1. Complete lines 52 and 53 as instructed.

   2. Enter “RC” and the amount you want waived in parentheses on the dotted line next to line 54. Subtract this amount from the total shortfall you figured without regard to the waiver, and enter the result on line 54. 

   3. Complete line 55 as instructed. You must pay any tax due that is reported on line 55.

  The IRS will review the information you provide and decide whether to grant your request for a waiver. 

"http://www.irs.gov/pub/irs-pdf/f5329.pdf