To create a template by uploading an HTML file:

1. Click "Templates" on the "Messages" tab drop-down menu
2. Click "New Template" button
3. Click "HTML Editor (Advanced)"
4. Fill in "Name" and other fields as applicable
5. Click "Upload HTML File" tab next to the "HTML" section in the page
6. Click "Browse..."
7. Select HTML file from your local computer
8. Click "Open"
9. Click yellow "Create & Preview" button

Learn more about templates.