Where/How should I report the excess deferral from paying too much into my 401k?
First, if you haven't done so already, you should contact the Financial Institution holding your 401(K) to request a distribution of the excess contributions. This should be done before the due date of your return, 4/15/2019 to avoid double taxation on the amount. Check out IRS Pub 525 for more details: https://www.irs.gov/pub/irs-pdf/p525.pdf.
Next, you should report the excess contributions on Line 1 of your 2018 Tax Return by following these steps in TurboTax Online:
Login to your TurboTax Online Account
Click "Take Me to My Return"
Click "Federal" from the left side of your screen
Scroll down to "Less Common Income" and click "Show More"
Select "Miscellaneous Income" and click Start or Update
Select "Other income not already reported on a Form W-2 or Form 1099" and click on Start
Answer the Question "Did you receive any other wages? Yes
Click "Continue" through the questions until you get to "Any Other Earned Income"
Answer "Yes" to "Did you earn any other wages?
Indicate "Other" as Source of Other Earned Income and click Continue
For the description enter "2018 Excess 401K Deferrals" and click on Done