Set up taxes by clicking Settings (1), switching to the Taxes tab (2) and clicking the small "+" (3).
The description (1) should help you to choose taxes when budgeting.
Percentage (2) is the tax percentage
Tax Refund in Percent (3) means how much of the tax can be refunded. In most countries this is 100%.
If parts of the tax are non-refundable, you can specify the budget account where these costs are budgeted. (4)
In (5) and (6) you can specify the intervals for tax payment and tax refund. These intervals are important for the cashflow.
If you are using producers in your budget, you can choose to set taxes automatically to all their costs (7)
To assign taxes to an account, select it (1) and then choose a tax from the macro area (2).