Circle wrote up an excellent guide on connecting the two accounts.
Additionally, this video does a great job of showing you how to setup your account if you're more of a visual learner.
Once you've connected your account you can create Circle rewards by navigating to the REWARDS page and clicking on the ADD A REWARD button. From the suggested rewards list you can pick either the Extended Screen Time or the Extended BedTime reward.
You will need to keep the reward names exactly Extended Screen Time or Extended BedTime for them to work with Circle. The rewards are also locked at extending the time by one hour.