See this article for more detailed instructions: https://www.gliffy.com/user-manual/?productId=3#article_453
Here are the steps for using an existing Gliffy diagram as a Template diagram:
Login to your Gliffy Online Account (if already logged in, clicked File, New)
Click "Use your own File as a Template" File, New
- Navigate through your folders to locate the document
- Double click to select (or single select and click "Create Document")
- Remember to save and name your new diagram!