Here is a discussion from bogleheads
https://www.bogleheads.org/forum/viewtopic.php?p=7731023#p7731023
To summarize:
1. Do direct Roth conversion and withhold enough in taxes to avoid IRS penalties. Withholdings are always timely.
2. From your taxable account move money into your Roth IRA that is equivalent to the amount of taxes withheld in step #1. Indicate that this is an indirect rollover. It cannot be done online at Fidelity or Vanguard. You have to call them.
There is no 12 month restriction.
There is no penalty for being under 59.5.
All money is converted from IRA to Roth IRA.
All taxes are paid out of taxable account.
Withholdings are always timely.
No estimated tax payments needed.