1) Custom Post Excerpt
When adding a new post you might not see the Excerpt box showing to add an Excerpt for the new post you are adding.
In the top right corner of adding a new post you will see the button Screen Options. Click this box to open the list of post boxes you’d like to show/hide when creating new posts.
You will now see the list of boxes that are available and you can select which boxes you want to show or which boxes you want to hide.
Enable the Excerpt box and you will then see it appear in the post below.
Excerpts are optional hand-crafted summaries of your content that can be used in your theme. Learn more about manual excerpts.
Useful plugins for editing the excerpts:
2) Using the More Tag
To place the More tag… you can place the cursor where you want it to display the link to the full post and simply insert it by clicking on the more tag button.
3) Using our custom shortened content setting (Premium Feature)
We’ve added some custom functionality to add shortened content to your blog post list. This setting strips the HTML tags in the summary, so bold/italic and other styling tags will not be displayed in the summary.
Otherwise, we do recommend installing this Advanced Excerpt plugin to further customize your Excerpt to how you want it: