How to designate max Fed withholding in paycheck, with 2020 W4 (allowances no longer exist)

I mean I want ALL of the remainder (after all other deductions) of my paycheck withheld, so that I have $0 net pay (after all deductions, including the Federal Income Tax deduction that I want to max out).

How do I instruct this on my 2020 W4?

This appears to be how to do it prior to 2020:

claim zero allowances on line 5 of your W-4.

You have a misunderstanding about the old W-4 forms :

• Claiming zero allowances on the old form would have the "maximum" amount withheld but it wouldn't get get you net pay to zero. Back when there were personal exemptions and lets say they were worth $4K, then every allowance would decrease your taxable amount by $4K a year or $153.85 per two-week paycheck. This was used to lower your withholding if you you had dependents, mortgage interest and the like. That $153.85 decrease would translate into less tax withheld, but the exact amount depended on your where your last dollars fell in the tax table.

  Claiming zero just meant that you would have approximately the right amount withheld if you worked all year, had steady income, and took the standard deduction with no kids.

• to have more withheld you put a $ amount in line 6:

6. Additional amount, if any, you want withheld from each paycheck.

You would have had to experiment with how to put a number here to get your net pay to zero. I say experiment because I have no idea how the system would react if you said hold and extra $800, and there was only $777 of net pay before this line. It would be tough to get exactly right if your pay wasn't steady.

In the old forms this was done when you had multiple jobs to make sure that you had enough withheld.

That means that your guess regarding the new forms:

My best guess is to designate/enter an amount on 2020 W4 line 4(c) [for "Extra withholding"] that I know will be greater than [or equal to] my net pay per paycheck (the net pay I would get by default if I don't submit a W4) - would that indeed be how to do it?

That would appear to be correct.

This questions leads to wanting to know why you want a paycheck with zero net pay. If you are trying to have a ton withheld because you know you will will be writing a big check to the IRS, then you don't need to get the amount you owe down to zero. You only have to get within one of the safe harbors, or you can file a quarterly estimate. Depending on how much you need to have withheld, and the small number of paychecks left in the year, the safe harbor might be hard to reach and the quarterly estimate may be a better alternative.

also:

(the net pay I would get by default if I don't submit a W4).

If you don't submit a W-4 they will withhold as a single, which will have more withheld than if you check married. The old form actually had an option for this you could ask to be considered "Married, but withhold at higher Single rate." that option appears to be gone on the newer forms.