It's a very painless process.
- If you are not logged in to GoDaddy, you login using the right GoDaddy customer account - either one which you currently own, or your new account, setup by "Buy a domain".
- You use the Blogger / GoDaddy "Update your DNS records" wizard.
- You provide the URL of your domain, to the wizard.
- You're done.
If you can't setup your domain, by using "Buy a domain" - or if you need to reset your DNS addresses, for a GoDaddy hosted domain.
- You already have a GoDaddy hosted domain.
- Your GoDaddy hosted domain registration just expired.
- You just transferred your domain to GoDaddy.
- You purchased your domain using "Buy a domain", and you now need to reset the DNS addresses.
So, we see one more reason to purchase your domain from GoDaddy.