This is the config file located at /etc/ddclient.conf:

# created by Brandon Smith
use=if, if=enp0s3
ssl=yes
protocol=googledomains
login=$LOGIN
password=$PASSWORD
downloader.local.bismith.net 

Of course, replace $LOGIN and $PASSWORD with the credentials provided by Google Domains for the domain or subdomain that you’re setting up. Also, make sure that the last line is your FQDN.

You’re probably going to want to set this up as a daemon in Linux. Edit the file /etc/default/ddclient, change run_ipup to false, and change run_daemon to true. After this, you may need to run sudo systemctl restart ddclient.service.

My above use case is for a machine that is reporting its local IP address, despite being behind a NAT. In my case, that’s what I want, but you may also want to report the public IP if the machine runs a server that is configured with port forwarding to be visible to the internet. Many examples of this can be easily found by Googling around.