From The Document Foundation Wiki
How do I create my own templates?
- Create a presentation that can be used as a template (including any master page customization you need)
- When the template document is complete, select ▸ ▸
- Choose the category in which to save your template and give it a name
When you create a new presentation, you will find your template in the list of available templates.
If you want to make the template available to all users of the machine, you must save it in the category Presentations (requires write permissions).