How do i document of form 8606 a non-deductible $8,000 to traditional IRA with Turbo tax records reflecting non-deductible proportion for subsequent RMD distributions?

You will enter the nondeductible Traditional IRA contribution for 2024 in the IRA contribution interview:

1. Click on "Search" on the top right and type “IRA contributions” 
2. Click on “Jump to IRA contributions"
3. Select “traditional IRA”
4. Answer “No” to “Is This a Repayment of a Retirement Distribution?”
5. Enter the total amount you contributed for 2024
6. Enter the amount contributed between Jan 1, 2025 and April 15, 2025.
7. Answer “No” to the recharacterized question on the “Did You Change Your Mind?” screen
8. Answer the next questions until you get to “Any Nondeductible Contributions to Your IRA?” and select “Yes” if you had a nondeductible contribution before this tax year.
9. Enter your basis in the Traditional IRA from your 2023 Form 8606 line 14 (if you had a basis in the prior year)
10. On the “Choose Not to Deduct IRA Contributions” screen choose “Yes, make part of my IRA contribution nondeductible” and enter the amount (if you have a retirement plan at work and are over the income limit it will be nondeductible automatically and you only get a warning and then a screen saying $0 is deductible).

You must tell TurboTax you made a contribution, even if it is "in the mail" still,

your custodian must know you intend a 2024 contribution.

@rgoren