In you don't have a candy or frying thermometer, don't worry. Just drop a (dried off!) slice of potato in the oil, if it sizzles immediately but gently, the oil is the right temperature. If it doesn't sizzle or sizzles slowly, the oil is too cold; if it sizzles violently, the oil is too hot.
Add a single layer of potato slices to the oil. Fry the potato slices until they are golden or brown, as you like, on both sides, about 3 minutes per batch.