Preheat the oven to about gas mark 6 /425F (200C). Whilst the oven is heating melt the lard or heat the oil in the oven, so there should be about 1/4 inch of fat in the pan. Then add the cut up potatoes and leave for around 45 mins, up to an hour if they need it.Some people claim you need to boil the potatoes first, I never have done so.