Open Google Document, click "Add-ons" menu, click "Get add-ons..."
On the popup, type "Secured Signing" from the search box. The "Secured Signing Add-on" should Add-onear in the result list. Click "Free" button to connect Secured Signing Add-on with your Google Doc.
The Google Doc takes you to an authorization page to allow the privileges that needed in Secured Signing Add-on. Choose your Google Account, the click Allow button to finish the authorization
Once Secured Signing Add-on connected, an initial popup should be shown.
Next: learn how to use Secured Signing Add-on