Adding a read more button
The "read more" button is created by inserting the "more" tag in the content editor of your post. It's part of WordPress default functionality. The button for it is located on the top row near the end, right after the "unlink" button. Place the cursor in your content editor where you'd like the read more button to appear and then click the "More Tag" shortcode button to add it.
That will automatically create the break between the post overview and single post with a "read more" button that links the two. More information here: