That being said, many of the domain registrars out there love to either through lots of extras at you (GoDaddy) or often don't provide the tools that you need to manage your domain effectively. Because of this, we've arrived at Namecheap as the domain registration provider that we use and recommend (we don't receive any compensation from them). Their prices are good, their tools are robust, and there is very little upsell.
The problem? Their interface is clunky at best. So what we've done here is to walk you through the steps, visually, for how to set up your Blogger blog's custom domain with Namecheap after you've purchased your domain.
Step 1: Go to the Manage Domains page
You do this by mousing over Domains in the top navigation and going over under My Account to Manage Domains.
Step 2: Click on the domain from your domain list that you want to manage
We obviously have a long list here, but simply click on the domain in the list that you want to manage.
Step 3: Click on All Host Records in the left navigation
Step 4: Enter the following domain records into the list
Step 5: Go to the Settings page in your blog's Dashboard, click on "Add a custom domain" under the Publishing section, and enter your domain
Step 6: Get the random CNAME value that Blogger now provides (highlighted in the picture) and use it in Step 4, then SAVE your entries in Namecheap
Step 7: Save your entries in Blogger and it should now be set up for WWW
The now extra set is to Edit the entry you just saved in Blogger and to click the checkbox that allows you to "Redirect yourdomain.com to www.yourdomain.com" and hit Save.