In most cases, it shouldn't be necessary to calculate estimated tax on RMDs.  But if RMDs are a large part of your income, you could do it like this: Take the Dec. 31, 2020 balance in your account and divide it by your life-expectancy factor; most people can take them from the table at https://smartasset.com/retirement/rmd-table.  That gives you the amount of your RMD.  Multiply it by your marginal tax rate (find your bracket at https://www.kiplinger.com/taxes/tax-brackets/602222/what-are-the-income-tax-brackets-for-2021-vs-202...), and that gives you an estimate of what your tax will be.  Do not pay it now; it is not due till you file your 2021 tax return in early 2022.

you have a choice;

calculate the tax for 2021 and then pay in 90% of the tax in quarterly installments 

or

since your 2020 tax will probably be lower than your 2021 tax pay in 100% of the 2020 tax in quarterly installments. if your adjusted gross income was more than $150,000 in 2020 you'll need to pay in 110% of your 2020 tax under this method.  

under the second method you may end up owing a lot come filing time 4/15/2022. if this worries you once you know the required quarterly payments you can always pay in more.