You need to set them up in the Google Home app and they must be on the same lan as your hubitat hub.
I would also recommend that your reserve ip addresses for each device in your router.
Then go to the apps page and install the built-in chromecast app.
As part of the installation, the app will search your local lan and, once found, it will list any gh minis that it has found.
Be patient because the detection can take a few minutes before the app finds your devices.
Then just select which devices you want to connect to hubitat.
The app will then create the GH devices for you and put them into your ‘devices’ page