Open your Gmail account.
If you do not have your Gmail bookmarked or pinned, an easy way to have access to all things Google is to go to google.com and click on the "waffle" in the top right hand corner.
Select the "Labs" tab in the setting menu.
This option is an experimental lab in Gmail. That means that Google is trying it out as a feature, but it hasn't become an official or permanent part of Gmail yet. Just know that all labs could change or disappear at any time.
To enable Preview Pane, click the Labs tab and scroll down until you see the "Preview Pane" lab. Click the Enable button.
Be sure to scroll all the way down the page and select "Save Changes".