You find it in the Home app under Settings, Works with Google. Just search for TRIGGERcmd.
After activation, you'll have a virtual switch for each of your commands that have a voice field filled in. Your computer also needs the voice field filled in unless it's your default computer. Your first computer is automatically your default computer.
This YouTube video shows how it works.
To see a list of sentences that will run your commands via the TRIGGERcmd Smart Home action, see the Smart Home Voice Commands page.
Use Google's script editor (not the Home app) to create automations that use TRIGGERcmd virtual switches. The switches can be used as an action to run your command, or as a starter to run the routine when your command runs.
If your virtual switches are offline, change the "Device type" to lights, then change it back to Switch again.