To set up a new Sales Tax Code you will first need to select the appropriate regime, you should be able to identify these by searching through either the Tax Code or the Description.

When you open an unconfigured Tax Regime you will see a window similar to the below where the Tax Code box at the bottom of the page is empty.

Selecting the New Sales Tax Code button will create a new window allowing you to start configuring a new tax code.

Here you will need to fill out the following information:

Sales tax code: The code you will be using internally to define the Tax Code such as “EXEMPT” or “STANDARDVAT”
Description: A short description of the 
Supplier only: Check this box if the tax code will only be used within Supplier Orders, otherwise this will appear on all Transaction Documents.

Once this has been completed you can start adding in the tax rates by using New Sales Tax Rates

Rate %: Set the Tax rate that this will be charged at.
Valid From: Set when you want to start charging this rate from, this will affect any Transaction Document if the Document Date is changed and is after the Valid From date.
Valid Until: If the tax rate changes you can expire this rate by setting the Valid Until date and then creating a new Sales Tax Rate.