You'll need to be signed into your Food52 account in order to use your Shop credit. As long as you're signed in, you should see this amount automatically slip off your bill at checkout. Because your Shop credit is tied to your account, any balance you have leftover will automatically roll over to your next order.
Please note that for security purposes, you’ll still need to enter and save a credit card & billing information at checkout.
If you'd ever like to check your Shop credit balance, just visit your Shop Credit page.