After an invoice has been generated you can opt to select what specific items in your invoice can have tax applied to it. First, you must toggle to "Edit Mode". You can simply click on the button "Switch To Edit Mode."
Once in "Edit Mode" you will see boxes to the left of each line item. Selecting each box will add the items to the total taxable value within the invoice. Further down to the right, you will be able to enter the Tax Rate which will compute the total Tax Amount by multiplying the tax rate against the total taxable amount selected.