It will open in a new tab of your internet browser. In the new tab, click on the email’s options menu and select print.
Remember to open the options menu for the individual email you wish to print, rather than the menu for the entire email thread (clicking “...” to the right of the email subject won’t give you the option to print). To print the entire email thread, simply print the most recent email in the conversation, and all previous emails will be included.
You can also save an email to a .PDF using these same steps.