there is of course more than enough information on templates at wikipedia templates but what you are looking to do is to just add or the like to something you are already familiar editing with to put two historical populations tables together you would use a table help table to put template historical populations into the template space for a city specific table you would be using template historical populations as a meta template which it is currently not designed for i suggest asking at template talk historical populations that it can be used in the template space by adding or similar to it