penalties can be avoided for 2021 if your timely withholding and estimated tax payments are 90% of 2021 tax or 100% of 2020 tax (110% if your AGI for 2020 was over $150,000 or $75,000 if MFS).
timely means
25% by 4/15/2021
50% by 6/15/2021
75% by 9/15/2021
100% by 01/15/2022 - withholding for 2020 can only be through 12/31/2021
withholding is assumed to occur evenly throughout the year but a taxpayer can choose to apply withholding to the period it was actually paid. 2210 must be filed and box D in part II
there is also no underpayment penalty if the tax due for 2021 after subtracting withholding, but not estimated tax payments, is less than $1,000 or the taxpayer was a US citizen or resident and had no tax liability on a 2020 return that cover 12 months, ie citizen or resident for all of 2020
you can also use the annualized income (AI) method in form 2210 to compute what must be paid in for the 3,5,8 and 12 months. see form 2210 and read the instructions for this carefully because the AI method is tricky. if you use the AI method for one period it must be used for all periods
https://www.irs.gov/pub/irs-pdf/f2210.pdf
https://www.irs.gov/pub/irs-pdf/i2210.pdf
