Get a bowl of cold water ready. Cut your potatoes into thin slices, about 2.5mm thick. Then, cut each potato slice into 2.5mm sticks, so that you have thin, baby fries - a super-sharp knife will really help. Transfer the potato matchsticks to the water as you go and leave them there until you're ready to cook.
Pour enough oil into a deep, large frying pan so that the oil is about 2.5cm deep, and place it over a medium-high heat. It's hot enough when the tip of a fry sizzles slightly. You don't want the oil to be too hot because you want the fries to cook and crisp up, not instantly burn.
Drain off your fries and carefully add about 2 handfuls of them to the pan. Leave them to cook in the oil, turning occasionally, until they are mostly golden (about 3-5 minutes). Drain them on kitchen paper while you fry the remaining handfuls, then season with salt. (Warning: try to avoid eating each batch immediately, before you've fried the rest.)