One of the great features of macOS is the ability to use almost any file as a template for creating new files based on that file. Usually you would use something like Word to create a letterhead, invoice etc and then save it as a template so that you have a base to work from for all future letters, invoices etc. However if the application you're using doesn't allow you to create a template, this isn't an option. Luckily macOS has this feature built in and it's called Stationery pad.
The first step is to save your file as normal, in this example we're using TextEdit which doesn't usually have a feature for saving templates.
Once the file is saved, locate the file and right click and click on Get Info (or click once on the file and press CMD + I).
In the info window that appears, click the 'Stationery pad' box and then close the window.
Now when you double click the original file, TextEdit will automatically create you a copy and this is what you'll be working in. You can then rename this file and the original file is still safe and unchanged.
This works for most editable files on the Mac, Word files, Pages, TextEdit, image files etc. You may find that some files don't allow this feature and you'll see that the Stationery pad is greyed out on those files.