To change your nameservers at Google Domains, start off by heading over to:
From here its only a couple clicks after logging in.
After logging in, click 'My Domains' on the left hand side
Now on the left hand side, click the DNS menu as shown here:
Now in the middle of the screen fill in the circle next to 'Use custom name servers'.
You will now be able to fill in your nameservers, start off by clicking the + sign to give you an additional field for your 2nd nameserver:
You can now enter in your Nameservers and proceed to click 'Save'.
This change usually takes a few minutes to a couple hours but can technically take up to 48 hours.