First step is to log out of your Hallow account by following the steps here:
Next, go to https://hallow.com/portal and log in to your account
*Note: If you already have a paid subscription, you cannot apply a gift card to your account
Then tap on "Subscriptions" on the left and tap "Purchase" a subscription
Scroll down, enter the promo code (you'll need to find this in the email you received as it's different for each gift card), and click "Redeem"
Sign back into the app on your phone and you should be all set!
If you have any questions at all please reach out to us here or at support@hallow.app