Open the Google Developer Console: https://console.developers.google.com
click the "Select a project" selector, then click: Create a project
Give your project a name and then click "Create"
Now you need to enable an API
The recommended API for Authentication is Google+ ("Plus"):
As if you had not clicked enough, you still have to click "Enable API":
Then click on the "Credentials" link in the left-hand menu.
This is the info that will be displayed for people using your app when you ask them to Authorize your app to use their Google Account:
This part is important as the redirect URI will need to match
what you define in your app's REDIRECT_URL option.
Once you have done that you will be shown a modal with your client ID and client secret:
Copy the two keys and export them in your project see: Step 3 of the Readme.