This answer illustrates the procedure for LibreOffice 5.1.4 (using the "Galaxy" icon theme) on Ubuntu 16.04 with Unity Desktop and consists of 4 parts:
- Step 1: Create / load the document you want to use
- Step 2: Store your document as a template
- Step 3: Configure the new template as default
- How to reset the default template?
Step 1: Create / load the document you want to use
First, create your template like a normal LibreOffice Writer document. You don't even have to save it as ODT anywhere if you don't want to - but you can do, of course - or just load any document.
I've just thrown together this stupid simple page as an example for now:
Step 2: Store your document as a template
Navigate to the File → Templates menu and click Save as template...
A Template Manager window with a Save button will open. Just select the My Templates category (or wherever you want to put it) and click Save. Enter a name for your template in the appearing dialogue window and hit OK.
The Template Manager window closes and your template got stored successfully.
Step 3: Configure the new template as default
Now you want to set up your template as default, so that it automatically loads when you launch LibreOffice Writer.
To do that, navigate again to the File → Templates menu, but click Manage this time. Alternatively, the keyboard shortcut to open the Template Manager is Ctrl+Shift+N.
Open the category you stored your template in and select it. Then you can click the Set as default button from the Template Manager's toolbar. Nothing obvious will happen, but your template got set as default now.
Close the Template Manager. The next time you restart LibreOffice or create a new empty document, it will be based on your new custom template.
How to reset the default template?
If you want to undo your changes and get the default template (plain white page) back, simply open the Template Manager again, as described above.
Click on the little cogwheel button in the right side of the Template Manager's toolbar and select Reset Default Template → Text Document from the drop-down menu.
If there is no Text Document entry in the Reset Default Template menu, this simply means that you already use the default text document template. Other entries like e.g. Presentation may also appear there in case you have set a template for in that case LibreOffice Impress presentations, but we don't have to care about that here.
That was it, you're back to the roots.