At the end of each blog post you will see the related posts section. This automatically displays 3 or 4 (depending on the theme) random blog posts from the same category as that particular post.
However, you can override this is you wish and manually select the related posts that you wish to show.
To manually select the related posts for a blog post, start by opening up a blog post and scroll to the bottom of the page. Here you will see a section titled Related Posts with the option to choose up to 3 or 4 (again, depending on the theme) related posts.
Simply click on the dropdown list and select the blog post that you would like to display. Once you have chosen a post, the random related posts that were showing will automatically be removed.