To set up an OAuth 2.0 client, navigate to "APIs & Services" and select "OAuth consent screen."
For most installs, you should select “External”, the “Internal” option can be selected if you are deploying Library with a google account managed by your organization (a paid account).
After selecting the user type, hit “Create.” The next screen will have several fields on it, but you only need to worry about one right now; the Application Name.
Fill it in with something applicable to your Library site and hit “Save” at the bottom of the screen.
On the next screen, click “Credentials” in the sidebar. (If you’re at a narrower viewport, you can also find this in the menu under “APIs & Services”)
Click “Create Credentials”, but this time select “OAuth client ID.”
Select "Web application" for the Application type and add a descriptive name.
Press "Create" at the bottom of the screen. You might find it helpful to leave the popup open as you move on to the next step, but you can also get them back later if necessary.
You're finally ready to deploy to Heroku!