First of all, the 250 rule is done to research about your niche.
We understand it won't add up to 250 if you enter a small city. You need to make sure that when you scale to bigger cities, you have at least 250 people searching for it.
So to do it properly, select a major city. Do your research, get the best niche and keywords, and then start building city pages. Start with smaller city pages first and then on to the major city.