The email template tool is built-in on the Sharing page. When you are on the Sharing page, simply click on Insert Email Template at the bottom of the message field to access this feature.
1. Click on Manage Templates or Create New
2. Give your Template a name, and start typing! You can access the message formatting tool by highlighting the text you want to format. Click on Save when you are done.
1. Click on Insert Email Template, and select the one you would like to use for this email. It's as simple as that!
Related: How do I share the gallery with my client?