Bring a 20cm or 8" saucepan of generously salted water to a boil (I use 1 teaspoon of salt in a pan this size).
Peel and cut the potatoes into large 40-50mm (1½-2") pieces.
Drop the potatoes into the boiling water and cook for 10 minutes, then remove them from the water, toss them around in a colander to rough up the edges and allow them to steam dry for 10 minutes.
If you are cooking in a separate tray, melt the fat in a pan, toss the potatoes in it and drop them in the oven for 60 minutes at 180°C or 350°F.
If you are cooking in a pan with a joint of meat (I think this is the best way). When the meat has 45 minutes left to cook, add the potatoes, spoon over the fat from the base of the pan and cook until the meat has cooked and rested. If your meat is cooking at a temperature below 180°C or 350°F, cook the potatoes for an additional 15 minutes and turn the heat up to 180°C or 350°F when you remove the meat for resting.