To highlight entire rows or columns in Excel using conditional formatting, you should do the following:

1. In the desired sheet, select the range that should be formatted conditionally.
2. On the top ribbon, click on the Home tab.
3. In the Styles group, click on Conditional Formatting.
4. Choose a formatting style from the drop-down menu.
5. Choose the formatting criteria from the drop-down menu.
6. Click OK and you will see the conditional formatting in effect.