Inserting `rel=nofollow` links inside your posts or pages via shortcode is an easy and much better method of inserting no follow attribute because here you can decide what links you want to prevent from search engine crawl.
So first of all install and activate Nofollow Shortcode plugin. After activation the plugin registers a nofollow shortcode which you can manually use inside your content and make your links nofollow.
How To Use NoFollow Shortcode ?
Insert [nofollow url="your-link-url.com"]Link Name[/nofollow] inside your post or page replacing your link’s information in the given shortcode.