Create a new bucket for Rudder to store staging files
Create a new service account under IAM & Admin -> Service Accounts
Add Service account permissions:
Grant access to write objects to the bucket
Grant admin access to BigQuery
Create a key for the service account with JSON type and store it
Create a new destination of type "Google BigQuery"
Add your project ID and bucket name
Copy the contents of the credentials file you created in the credentials section
Start sending events to your source and Rudder dumps them in BigQuery periodically.