Once you make a nondeductible contribution to an Trad IRA you can then convert it to Roth right away without paying tax, provided you don't have an existing Trad IRA so your balance consists entirely of the non-deductible contribution (basis). This "backdoor Roth' process is used by high income folks who can't contribute to the Roth directly.
All of this (tracking your non-deductible contributions / basis in the IRA, and tax calculations for Roth conversion) occurs on Form 8606.
Fidelity and Schwab usually have good information on these topics, see
https://www.fidelity.com/learning-center/personal-finance/backdoor-roth-ira
