There are 2 ways of doing do:
1. From the order confirmation page, you'll be able to either create an account, or if you have an account already, combine it, where it's shown below:
2. After you've placed your order, you'll receive an email confirmation for this guest checkout order. From this email confirmation, you'll be able to click the link to view the order's details and, from this link, you can create/combine to your account.