Remember, how you setup the custom domain has effect upon how soon it becomes usable, and how usable, overall, it will be for your readers.

 

The simplest way to setup a custom domain uses Settings - Publishing - "Buy A Domain For Your Blog".  But there is a builtin 3 day waiting period before the custom domain is fully operational.  And "Buy A Domain" is for new domains only - if you are publishing a blog to an existing domain - one with services like email, FTP, or maybe an existing web site, you'll want to involve Google Apps in the setup process.

 

With Settings - Publishing - "Advanced Settings", or with Google Apps, you can shorten the waiting period between setting up the DNS and having an active custom domain published blog.  You will have to be aware of how custom domains work, if you're going to go this way.

 

Every custom domain URL (primary or secondary) requires 2 settings.  With "Buy A Domain", they are made for you.  When you use "Advanced Settings" or Google Apps, you have to make each setting on your own.

• A pointer from a DNS server, which you have to pay for, to Google.  Generally, this will be a "CNAME" referral to "ghs.google.com".  Using "Buy A Domain", the "www" alias ("www.mydomain.com") becomes the primary URL, and the primary domain ("mydomain.com") will point to a trio of Google Apps servers, and becomes the secondary URL.
• A pointer from Google to the blog itself.  The pointer for the primary URL is setup when you publish the blog to the custom domain URL.  For the optional secondary URL, you select "Redirect mydomain.com to www.mydomain.com.".

Without these 2 settings, the URL just won't work.

 

And here is a final bit of advice, and a plea. If you are able to use my documentation, presented here and in my blog, to solve your problem, that's great. But note that the values "myblog" and "mydomain" are hypothetical examples, provided for use in the case studies. When you setup the blog or work on your problem, substitute the actual BlogSpot and custom domain URLs. Please, do not post in Google Blogger Help: Something is Broken, a report

My custom domain, "mydomain.com", doesn't work.

Give us the actual URL of the BlogSpot and Custom Domain. Help us to help you, more effectively.

For more reading, see The Real Blogger Status: Custom Domains Setup.