So go on their site and register a domain.Just make it forward for now we'll set it up later.

After you've signed up with them and confirmed your account you can use the domain you just registered.It will basically cover up " yoursite.blogspot.com " with " yoursite.tk " . Only problem is you won't be able to link to a specific post the link will remain yoursite.tk. If you're ok with that you can stop here.

To configure your .tk domain on blogger go to your account on dot.tk and find the domain panel.
Copy the settings in the image below only replacing the Host Name with the domain you've just registered.
We're done with that so we can close out of it.
Go to your blogger control panel , navigate to your blog go to Settings then Basic , find Blog Address and click edit.Click Advanced Settings.Enter the domain you registered and click Save.
 Your domain should go up in about 10 to 15 minutes.