By creating a Voucher Product on the Products page. Here you can:
Enter Voucher info
Add a Price Table (but no prices)
Attach Delivery Methods
Depending on what you want your customer to receive after refunding to a voucher, it may be worth either:
Creating a refund/voucher-specific Delivery Method with Email/Ticket Templates containing Voucher info and Merge Tags - so you can attach it to your Voucher product.
Or adding Voucher/Product Merge Tags onto your standard E-ticket Delivery Method.