"I have looked at the form instructions and they seem a little vague"

By withholding = from Payroll on a paycheck dated in that tax year.

By estimates = by the final due date for estimates for that tax year, such as Jan 15 is for the prior tax year.

You are asking about Safe Harbor. There are three tests and you need to know what applies: 90%, 100% and 110%. That's why you read the IRS reference materials.