There are two ways to connect Google with Smartnest:
Go to your Google Home App and add tap on the Add button, then select Set up device and select the option "Have something already set up".
Then search for the Smart nest Action and follow the linking process.
You can check the action on the Assistant directory
You don't have a Google Home device but you want to control your devices using Google Assistant you can configure the IFTTT integration and activate Google Assistant channel as a trigger and Webhooks channel as an action:
Don't forget to Add your Webhooks key to your Smartnest Account