How do I pay taxes to the IRS when I do an IRA to Roth Conversion to avoid penalties if I pay taxes only in April?

You can send in an estimated payment.

Here are the blank Estimates and instructions…..

http://www.irs.gov/pub/irs-pdf/f1040es.pdf

The 1040ES quarterly estimates are due April 15,  June 15, Sept 15 and Jan 18, 2022.  Your state will also have their own estimate forms.

Or you can pay directly on the IRS website https://www.irs.gov/payments

Be sure to pick the right kind of payment and year.....2021 Estimate

thanks.