Once a Voucher is purchased, the customer will receive the voucher code in their email and/or ticket (depending on which template you have inserted the Voucher code Merge Tag).
Then when on the checkout page, the customer can select the Voucher Payment Method and enter the code in the box to redeem.
These are the steps:
1. Once a customer has added items to their basket and gone through to the checkout page, they will see the Voucher Payment Method as an option to select at the bottom of the page
NOTE - You will need to have added this as a Payment Method and enabled the Website Channel in order for this option to be available. Learn more - Sell Vouchers
2. The customer can then enter the Voucher code that has been sent to them and click Confirm & Pay