You don’t need to use IFTTT if you’re using google assistant.
Anyway, if you want to use IFTTT first of all I guess you need a public IP or URL, like on duckdns, and then in your webhook applet ( https://ifttt.com/maker_webhooks ) you will write something like:
I have some IFTTT Buttons on my smartphone home that I use to send direct commands like opening my house gate when I get home or turning off all the light, without using the HA web front. You just make those IFTTT applets and then you link them to an IFTTT button.
In that case, as I said before, you don’t need to use IFTTT. Just set up the Google Assistant component, set up your lights in Home Assistant, add the Home Assistant configuration to your Google Assistant app and then you are done.
After that you’ll have just to say, “Hey Google, turn on the lights”.
Inside the Google Assistant app you can also set the rooms in your house and then you’ll be able to say “Ok google, turn off the lights in the kitchen”