You can download the NextUp app from the Apple store by searching for 'NextUp'.
1. Log in with your existing account information (NextUp membership credentials work across all platforms apart from Amazon Channels)
2. If you don't have an account yet, you can choose the option to purchase one from Apple. You will need to enter your Apple ID before purchasing your subscription on your AppleTV.
After you sign up successfully, check your Apple ID's email inbox for an email confirmation. From there, you'll need to click the link in the email that takes you to a web page to set a password. Once you have your password set, you can use your login details to access NextUp on any other platform apart from Amazon Channels.