Where do I report the after-tax distribution relating to my MRD?

If you have contributed after tax dollars to your IRA, then you should have filed form 8606 for every year you have made nondeductible contributions.

In TurboTax after you have entered your forms 1099-R,

1. On the summary page Your 1099-R Entries, click Continue
2. Answer Yes to Any nondeductible Contribution to your IRA?
3. Answer the questions about the basis from line 14 of your last filed Form 8606 and the total value of all traditional, SEP, and SIMPLE IRAs