NOTE: You can make the configuration of auto-related posts if you use Blog Plus or Blog Extra extension only.
In order to enable related posts please go to Admin Panel > Stores > Configuration > Magefan Extensions > Blog, find Post View section and choose Related Posts subsection.
1. Make sure that related posts are enabled in the Display Related Posts selectbox.
2. In the Enable Auto Related Related Posts (Blog+) select box choose Yes to enable auto-related posts. The algorithm of post-combine works on posts content and title basis. That means the module find similar words which present in your posts and uses them to combine corresponding posts.
3. Enter words you don't want to be used for search of related posts in the Black Words For Auto Related field. You don't need to enter one letter words e.g. (I, 1, 2), extension already skip such words.
As far as related posts are generated automatically by cron you have to make sure that cron is enabled for your Magento 2 instance and wait some time after you have saved the configurations.