Your supply lines will move food automatically in either direction depending on what city needs the food more. You can control a city's needs by adjusting the stockpile level up or down when the city is selected. If a city has less food than it's been told to stockpile then it won't pass anything along until it's reached that stockpile level.

So, in general if you want to move more food to a particular city, make sure it's stockpile level is fairly high and the surrounding cities have lower levels.

I hope that helps and don't hesitate to follow up if you've got more questions.