Do you already have a blog on blogspot? Your blog will look more professional if your blogspot subdomain replace with your own domain name. If you intend to buy a domain, I suggest you buy a domain at GoDaddy, because GoDaddy provides domain custom facilities. So you do not need to use other hosting services such as dnspark to do custom domain (domain leads to blogspot). There are many facilities you can use at GoDaddy, as well as to create an email address with your domain name as email@yourdomain.com by connecting with existing facilities in the google.com
If you already have a domain at GoDaddy and are still confused for setting custom domain, the following are my experiences in setting custom domain at GoDaddy.
The first step please log in to GoDaddy and click the domain name you want to connect with blogspot (blogger)

After you click on your domain name, you will are directed to the domain setting as shown below:

then do click on the “More Settings” to get to the next setting menu

Then click “manage” as an example image that I put a red box at the top, then you will be directed to the Settings control like the image below:

In the first line in the picture above is an example of the CNAME that was created to connect my GoDaddy domain to a subdomain blogspot, you also have to make the same CNAME for your domain connect to blogspot. To create a CNAME as the above, please click on the words “Add New CNAME Record”. Then you will be directed to the creation menu CNAME, Please make my CNAME as demonstrated below:

If the data you enter is correct please click “OK” and within moments you’ll be listed CNAME. To this end custom settings for the domain at GoDaddy already completed. next please log on to blogger.com blogspot subdomain settings.

Please enter the blogger dashboard and go into the settings menu on the blog you want to navigate to the domain. You can look at the example image below:

then click on the words “Custom Domain”, then you will be directed to the next menu like the image below:

Then please click on the “Switch to advanced settings” and the menu to direct your domain that already have will appear, please see the example image below:

Please enter your domain name with www like you’ve made in this GoDaddy, please follow like the picture above. If you have correctly fill you please enter “data verification” and click “Save Settings”.
Now that you have successfully directing your blogspot subdomain to your personal domain, to test please go to your domain or subdomain you. if you are not a subdomain you then you will be automatically directed to your new domain name. If the error still occurs, this still happens caused during propagation. You can wait a while for your domain briefing perfectly finished.