Getting your backdoor Roth IRA right is very tricky. If done incorrectly, you can find yourself owing a ton of money and more than negating the value of contributions.

Here’s the best/most correct covering of the topic:

How to Do a Backdoor Roth IRA

Here is a higher-level discussion:
https://www.dadsdollarsdebts.com/2016/11/22/roth-401k/

Also look here for much more in-depth information:

https://www.kitces.com/blog/how-to-do-a-backdoor-roth-ira-contribution-while-avoiding-the-ira-aggregation-rule-and-the-step-transaction-doctrine/