Below demonstrates how your users can apply their vouchers while they check out.
1. While making the order, your customers get an option, 'Apply Voucher' in the order page.
Note: Admin > Settings > Vouchers > General > Not login available setting determines if 'Apply Voucher' option is available to guest users. Default setting is disabled for guest users, so the users not registered don't see this option.
2. In the next screen, the customers can either select a voucher or input the Voucher code.