You must provide an HTML file.
You must use Absolute links in your file references. This means using the full url: http://www.mywebsite.com/images/mypicture.jpg
If you use graphics, you must host them on your own server. Green Sheet will NOT host images for you. See absolute links above.
Keep the layout simple.
Make sure your email can tell your story WITHOUT graphics, as all modern email clients suppress graphic images. Have your graphics add value, when the reader hits their “Show Images” button.
Provide a subject line that will grab the readers attention.
Some modern email clients have “dumbed down” their ability to read and interpret HTML (I am looking at you, Microsoft Outlook), so the simpler you keep your email, the more likely it is to reach your audience intact.