If you still have an FTP blog in your account, you will see an alert on your Blogger Dashboard. Follow that link to the Migration Tool, then select the blog you wish to migrate off of FTP.
After you download a backup of your blog, you will be prompted to select a new URL for your blog: either at your own domain or at blogspot.com. If you wish to have your blog hosted at a subdomain of your domain, you will need to create the CNAME before proceeding; details for doing so are here.
Once you make your selection (and if necessary, create your CNAME), you will download a ZIP file containing a full archive of your blog, with updated pointers to your new URL included in the HTML for each post.
Download and extract the files from the ZIP file, then upload the files to your FTP host. Once that upload is completed, you will be done with your migration. You can now create new posts and edit old posts from within Blogger, and visitors to your old URLs will be redirected to the new location.
If you encounter problems with the Migration Tool, please file an issue in the Issue Tracker and a Blogger team member will investigate.