When notified via email, your title will automatically checkout to you. Sign in to your account, access your loans page under the account settings, and your available holds will be listed there for you to download to your device.
If you do not want to use the Automatic Holds Checkout feature, access your OverDrive account and adjust your personal settings. Then, when notified it is available to checkout, you will have 72 hours (from the time of delivery of the email) to checkout your title and download. Items not checked out in this time frame will then go to the next person in line.