By going through the normal purchase flow - but on the Payment page, selecting a Voucher Payment Method and entering the Voucher code.
You just need to make sure the Voucher Payment Method is created before you do this.
Follow these steps:
1. Create a new Voucher Payment Method
a) On the Settings page, click New Payment Method
b) Select Voucher Payment Type and enter the name of the Payment Method
c) Select the Channels you want it to be available for and click Submit
2. Use Gift Voucher for an order at Box Office
a) Go to Box Office page and go through the steps to add Tickets to the basket
b) Once you're on the final Payment page, select the Voucher Payment Method
c) Enter the Voucher Code underneath and click Pay & Update Basket Total