Unpublished Cards will not appear inside the Card Store and, as such, you cannot add them to your Automations in the conventional way.
First, make sure you have a Automation ready. It is recommended to have a Automation made purely for Development purposes. Once ready, follow these instructions:
And there you go, your unpublished Card is now on your Automation ready to develop!
Now that your scaffolded Card is on an Automation, you can start actual development on it!