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.