First, let's look at the design templates that are in PowerPoint so that it's clear what we mean by templates. The place you go for templates is the Slide Design task pane, shown in the picture.
The task pane has a gallery of thumbnails that represent the available design templates. All you do is click one of these to apply its design to slides in your show. Each template has a name and exists as a file on its own, with a .pot file name extension.
You'll build on one of these to create a new template that you'll add to this gallery. That way it's available to apply to a presentation whenever you want it.
Note The template you'll be creating is a design template rather than a content template. The latter, in addition to containing the design for the presentation, has multiple slides and includes text content. You find content templates in the AutoContent wizard (which you can open from the New Presentation task pane).
By contrast, the templates you find in the Slide Design task pane are design templates only. Their thumbnails do have writing on them, but that only represents text styles and not actual text content.