While AdSense for Domains is available for a long time, Google has been requested by publishers using AdSense for domains to make it easier to park currently unused/undeveloped domains with Google. Today, Google has simplified the process of setting up AdSense for domains.
"As of today, to get your domain setup with AdSense for domains, you can simply delegate your domains' nameserver to Google's DNS nameserver. This is typically a single step with your registrar, and simpler than the old method of changing A records and creating CNAME records. You can still use the old method if you prefer, and domains that are already set up will continue to work," explains Google.
To get started, review instructions for delegating your nameserver to use Google's DNS nameserver in Help Center.
More Info: sign up for AdSense for domains