Create or upload a document that you would like to template.

Navigate to the file in your Project via your Project Library. Hit the bookcase icon on the far left.

Here you can narrow down your search using file types etc. 

Press on the button with the three little dots next to the file and "Save as Template". Give the template a title and hit "Save".

When you next create a document, you can choose to use the template from the dropdown menu.