You will need to be logged into your admin so that you can set up your site so customers can use a gift card to purchase items with.
Once in the admin, hover your mouse over the menu item 'Settings' and then select 'Commerce'. You will then scroll almost all the way to the bottom of that page to the section that says "Gift Cards." To Enable Gift Cards for your site, you will need to check this box. This is just if you want customers to be able to use the gift card on your site. This is not required if you only want them to use them in-store.
Now you will go back to the homepage of your admin and hover your mouse over 'Shop' and select 'Gift Cards' in the drop down options.
To add a gift card to be used on the site, you will then need to add this via the Gift Card Menu. On the right hand side of this page, you will see an option for "Add Gift Card."
Next you will enter in your information on the gift card. If you want the customer to be able to use the gift card online, you will need to enter each gift card this way.
NOTE: This step is crucial, if you don't add the gift card in here first, your customer will not be able to utilize the gift card as it doesn't exist in the system.
It will auto generate a code for you but you may also insert one if you need a specific code.
Next, add the Currency Type, total Amount of the Gift Card and when it will expire (if it does). You will then save the changes and add it to the system.
You have now created and activated a gift card that your customers can use online to purchase with. The system will automatically update the balance of the gift card each time the customer uses it.