Open the Home app on your iOS or Android device, then slide out the menu from the left edge of the screen. From the list of options, tap on Home Control. Next, tap on the + button in the bottom right corner, then select the type of device you want to add to your Home account.
The app will walk you through the authorization process for the device or service, and then ask you to assign the device to a room in your home. Repeat these steps for each service you'd like to control with your Home device. With setup complete, you can use commands such as "OK Google, set the temperature to 72 degrees" or, "OK Google, turn on my lights."