It's quite easy!
- Log in to your account,
- On the subscription you'd like to set to auto-renew, MENU (see image 1 below) it will expand and you'll see the option to Auto-Renew Subscription (see Image 2 below).
Once that's done, your subscription will automatically renew the month after your last prepaid box. For more details on auto-renewal see here.
If you have any questions about this or you run into any trouble, please write to firstname.lastname@example.org so we can help you out!