1. Get a service account key from https://console.cloud.google.com/apis/credentials (You can here also restrict what this keys is allowed todo)
    1. When creating, make sure you click the Furnish a new private key
    2. Select JSON when it asks you how to download the key.
  2. The service account key you have just generated includes a client_email.
    1. Go to you google spreadsheet and allow this client_email to have write access on this document