penalty for underpayment of estimated tax

How will the underpayment be calculated, when is it determined, and when do I pay?  An estimated underpayment penalty is calculated by the program and paid with the return.

Generally, most taxpayers will avoid this penalty if they either owe less than $1,000 in tax after subtracting their withholding and refundable credits, or if they paid withholding and estimated tax of at least 90% of the tax for the current year or 100% of the tax shown on the return for the prior year, whichever is smaller.

If you decide to make an estimated tax payment for tax year 2020 it must be made on or before January 15, 2021.  Go to this IRS website to make a tax payment - https://www.irs.gov/payments

If the penalty is calculated then ...

You might be able to eliminate it or at least reduce it.  You can go to Federal Taxes tab or Personal tab, under Other Tax Situations and select Start by the Underpayment Penalties. You will answer a series of questions that may reduce or eliminate the penalty. Or you can elect to have the IRS figure the penalty for you.  It's form 2210.

It's under
Federal Taxes or Personal (for H&B version)
Other Tax Situations
Additional Tax Payments
Underpayment Penalties - Click the Start or update button

https://ttlc.intuit.com/questions/2547642-what-s-the-irs-penalty-for-filing-late

Can I just let the IRS notify me?  After you file any underpayment penalty you have not yet calculated will be billed by the fed/state.  

If so, are they prompt?  Sometimes.

The details of the calculation are a long and complicated subject.  But essentially, it's 5% interest on the amount you underpaid at each of the quarterly due dates.

If you don't pay the penalty with your tax return, then the IRS will send you a bill.  That's really the easiest way.  

However, there are several different ways of accounting for when your withholding was received and when your tax liability was incurred.  The IRS uses the basic method of assuming that all your withholding and tax liability was divided equally among the four quarters.

You may be able to get a lower penalty by using an alternative calculation, but you have to show your work to the IRS.  If you had more withholding in the early part of the year, then you can enter into TurboTax exactly when it was withheld, and possibly reduce your penalty.  Similarly, if you had lots of income late in the year, then you can show that you didn't need to pay earlier in the year, and reduce your penalty.

Side note: stop using the phrase "I neglected to..."  This has a specific legal meaning, that you didn't care enough about your tax obligations to do it correctly.  Don't admit to neglect, when no one has accused you of it.  You simply didn't pay your quarterly tax.  That's correct and sufficient, and all you need to say.

I DO NOT OWE ANY MONEY TO THE IRS FOR TAX YEAR 2000. HOWEVER I WAS LATE ON AN ESTIMATED PAYMENT. DO I HAVE TO FILE FORM 2210?