Follow these steps:

Create the template slide using Presentation
When the slide is complete, from the pulldown menus, select: File-Templates-Save
If you want to make your own folder for storing your personal templates (suggested), click on the "Organizer" button
In the dialog that appears, select: Commands-New
A new folder will be created and will appear on the right side of the dialog, named "Untitled"
Give the folder a new name and press the "Return" key to enter the name, e.g. My_templates
Click Close.
In the "Document Templates" dialog, on the right side, you will see a "Categories" list. Make sure that your new template folder is selected.
In the "New Template" field, enter a name for your template
Click on OK

Whenever you create a new presentation, your template will be available for selection.