If you've already created your OAuth credentials, you can modify your Redirect URI: Go to the Google API Console, click on the Credentials panel under APIs & auth, select Edit settings under your Client ID, then save.
Under the Consent screen panel, make sure you've filled out the "Product Name" and "Email Address" fields.
With your Google application in place, visit the Google > OAuth bundle. Starting with the InitializeOAuth Choreo, use these Choreos to grant your application access to a Google account. This short video demonstrates how to use our OAuth Choreos, and it's a good idea to take a break and watch it now before you dive into this step. This process will return a refresh token which can be used along with the client id and client secret to authenticate with Google.
Note that for API calls that do not access private user data, you can often authenticate with YouTube using the optional API Key input as well. Your API Key can also be found at Google's API Console.