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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If necessary, "Create Project"
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Click "APIs & auth" and "Credentials" on the left
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Click the "Create New Client ID" button
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Choose "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?close
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Click "Create Client ID"