Click on the button. The Tax Defaults will appear, as in the example below:
For each of the Tax Rates, click inside the Tax1 or Tax2 (or other tax) text box and enter the current tax rate.
For Taxable Items, click to place checkmark in the corresponding Tax1 or Tax2 boxes as they apply to each item. In the example above, only Tax1 applies to Ext. Warranty products, so the Tax2 box has been left empty.