Here's the first thing to come up on my Google search:

https://www.empower.com/the-currency/work/after-tax-vs-roth-401k

Note, these are not "excess" contributions, just after-tax contributions.

If done right, these are NOT included in the box 12 code D amount (you can have the client pull a final paystub to see the details).

You'll also have a 1099-R showing:

Box 1 = Gross distribution
Box 2a = $0 (or maybe a small amount of earnings)
Box 5 = Box 1 - Box 2a (same as Box 1 if no earnings)
Box 7 = G
IRA = NOT checked

Most of my clients that have this have "auto-conversion" turned on so as soon as they make the contribution from their paycheck it is immediately converted to Roth before there are any earnings.  Sometimes the Code G will throw off the software.  If there's anything in Box 2a you want that to show up as taxable on the 1040.  If that's not working, post back and maybe a Lacertian can help out with the data entry.

Rick