Enabling Google SSO requires that you have a Google account of your own. You will also need to restart your forum a couple of times to activate Google SSO for your users.
Install and activate plugin
Enable Google SSO
Authenticate your forum in the Google API console, which is linked from this page. Before opening the Google console, select and copy the text /auth/google/callback without the single quotes. You'll need this info later to link Google SSO with your forum.
Confirmation email address
You'll be asked to provide a confirmation email address and a product name. This information will be shown to users who register for your forum, so make sure the name given is descriptive of your site. On the last set up page, just click Done.
Your Client ID will now be set up. You'll need both Client ID number and Secret code to activate SSO. To get these, click on the name of your ID. The easiest thing to do is to copy and paste both these codes into a blank notepad window. Once you have done this, you can exit the Google API page.
Returning to NodeBB, copy and paste the Client ID and Secret code into the appropriate places on the Google menu page. You can also allow users who sign on with Google to skip the standard email verification step since Google usually does a good job of verifying these emails, but this is up to you. Be sure to save these settings. Once you are done, you will need to restart the forum one more time.
Once your forum restarts, new users will be able to join your forum using their Google ID. This option will be added to the user registration page automatically.