First, you'll need to get a Google Client ID. Follow these steps:
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Visit https://console.developers.google.com/
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"Create Project", if needed. Wait for Google to finish provisioning.
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On the left sidebar, go to "Credentials" and, on the right, "OAuth consent screen". Make sure to enter an email address and a product name, and save.
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On the left sidebar, go to "Credentials". Click the "Create credentials" button, then select "OAuth client ID" as the type.
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Select "Web application" as your application 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".