First, you will need to log into your https://www.123-reg.co.uk/secure/ scroll down until you find Manage Domains click on modify domain.
Select Manage DNS.
Add a new entry and you have to choose CNAME from the drop down list.
Now create a new CNAME record, enter "www" on the left box and "ghs.google.com." on the right, you are associating www.yoursite.com to blogger.
Now log in to Blogger and Select Settings tab then Publishing and enter your registered domain, click on save settings and wait 24 hrs.
Finally, after 24 hours we can reach our blogger site using our own domain name.
There is a couple of erros you may get during the first 48 hours:
1. You can access http://www.yoursite.com but not http:// yoursite.com.
2. When you try to access your blog you get a Server Error wait 30 seconds from Google.
There is nothing really you can do but wait, if you change the DNS settings on 123-reg more than once this will slow the process.