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 "Registered apps" on the left
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Click the "Register App" button
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Choose "Web application" as the type
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Click "Register"
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Expand the "OAuth 2.0 Client ID section"
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Set Web Origin to: {{siteUrl}}
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Set Redirect URI to: {{siteUrl}}_oauth/google?close
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Click "Generate"