Adding no follow tag to external links is always a good idea for any blog as it prevents search engine bots to follow external links that may not be relevant to your content. For example search engine bots follow links to social sharing sites via share buttons.
To add no follow tag to external links manually without any scripts or jquery codes just open your Blogspot template editor and find " <a " using 'ctrl+f ' . All links in your blogger template will be enclosed within these tags . See which links are external links and add rel="nofollow" just after <a for every link you want to be no-follow .