Whether you want www.yourdomain.com or subdomain.yourdomain.com pointing to a Blogger blog, the process is perfectly identical. First things first - add the following DNS records:
A www 126.96.36.199 A www 188.8.131.52 A www 184.108.40.206 CNAME www ghs.google.com
Of course you can replace 'www' with any subdomain of your choice.
Once the DNS is setup and pointing to Blogger, next step is to tell Blogger about your custom domain name. From the dashboard go to the Settings menu and choose 'Publishing':
Good! now www.yourdomain.com will point to your Blogger account and vice-versa!