There is of course more than enough information on templates at WikipediaTemplates 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 HelpTable To put TemplateHistorical populations into the template space for a city specific table you would be using TemplateHistorical populations as a metatemplate which it is currently not designed for I suggest asking at Template talkHistorical populations that it can be used in the template space by adding  or similar to it