It will depend a bit on from where and when your income comes from but in general, the same way you have calculated estimated taxes before. If you are relying on retirement accounts such as IRAs for example, you can usually just let the brokerage or bank withhold taxes from your IRA distribution rather than making quarterly estimated tax payments. For other types of income, each quarter, you should estimate how much of it will be taxable to check if your distributions and other income for that quarter pushes you into a higher bracket and make an estimated tax payment for that quarter if you need to "even up". One way that I do that is to create a new return for estimating purposes using last year's version of turbo tax and enter the income and taxes paid year to date into the appropriate income section section (1099-R for retirement income). Use last year's tax return to estimate deductions or simply assume a standard deduction.
Note that TurboTax also has a tool call Taxcaster. I have never used it but it may help give you an idea whether you are withholding enough or need to add more in a quarterly estimate.
