Login or register a new account at https://RegistryUI.com. Once established, visit User > My Subscriptions.
If your account already has one or multiple subscription, see Reviewing Subscriptions instead. Otherwise, if your account doesn't have a free subscription yet, the following is shown:
Click the "Add Free Subscription" button to add the subscription information to your account.
Once the subscription information has been added, it is automatically displayed so you can review it (see Reviewing Subscriptions).
The displayed Account Key and Account Token must be entered in your local RegistryUI instance to enable the subscription (see Entering Account Key/Token).