How to prepare a sample return for next year?

You mean for your 2021 return we do next year in 2022?  What version did you use for 2020?  It shows the Free Edition.  Did you have any self employment in 2020?

To plan for next year you can go though the Estimate section or in the Desktop program there is also a What-If worksheet. And in Desktop, you can save your return with another name to test in. Go up to FILE – SAVE AS. In the Online version click on Add A State to let you back into your return, Be very careful not to change any of your actual current return. Or you can just start a new return and new online account to test in.

You can just type W4 in the search box at the top of your return, click on Find. Then Click on Jump To and it will take you to the estimated tax payments section. Say no to changing your W-4 and the next screen will start the estimated taxes section.

OR go to
Federal Taxes or Personal (Home&Business version)
Other Tax Situations
Other Tax Forms
Form W-4 and Estimated Taxes - Click the Start or Update button

Say No to W4. When you get to the W4 and Estimated Taxes section, say you want to adjust your income to go though all the screens.