First up you'll need to make sure Dual Window is enabled:
Head into the Settings app
Under "Device," scroll down until you see "Dual Window"
Switch the on-screen toggle to "on"
While you're in the settings, too, you'll see an option for "Split view" that will automatically engage dual window when tapping on links in full screen or for image attachments in emails. If this is something that sounds useful to you, go ahead and check the box while you're there.
***Edited to add labels***
