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.
