First, you'll need to get a Google Client ID. Follow these steps:
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Visit https://code.google.com/apis/console/
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"Create Project", if needed. Wait for Google to finish provisioning.
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On the left sidebar, go to "APIs & auth" and, underneath, "Consent Screen". Make sure to enter a product name, and save.
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On the left sidebar, go to "APIs & auth" and then, "Credentials". "Create New Client ID", then select "Web application" as the type.
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Set Authorized Javascript Origins to: {{siteUrl}}
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Set Authorized Redirect URI to: {{siteUrl}}_oauth/google
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Finish by clicking "Create Client ID".