To create your own custom Slides templates, open Presenter > Slides > Create new and select an existing template as your base. Next, design the slide how you want your template to look. When you're happy with the template, right-click on the thumbnail of the slide from the right slide deck and click "Save as Template." You can also create templates from existing Slides by following the same "right-click > Save as Template" steps.
To create templates specifically for Song and Scripture service items, please take a look at this article.