After you file

as of this date there is no clear answer for self-employed indivduals to pay their deferred SE tax. supposedly the IRS will send out notices.

as a pure guess because currently the IRS Direct Pay methods don't provide for designating a payment as being for this purpose either the DP system will be revised so the designation is possible or there will be a form in the 2021 tax return where you designate an estimated tax payment as being for this purpose.

 

  

 

for employer's they file 941's and pay using the methods mentioned

https://www.irs.gov/newsroom/what-employers-need-to-know-about-repayment-of-deferred-payroll-taxes#:...