I've copied this basic recipe from a site, it's the easy way to do it! I sort of agree with it, I've put comments in where appropriate, based on experience. The whole point about Roast Potatoes is that they should be crisp, tasting of whatever meat you're cooking, yet soft on the inside.
1. Pick floury, not waxy. There are essentially two types of potato – waxy ones are good for salads, while floury make great roasters. Go for King Edward or Maris Piper. Go with King Edward - I get good results with Maris Piper, GREAT results with King Edwards
2. Boil until soft. Bring up to the boil in cold, salted water (I steam all veg except when I'm doing potatoes for roasting or to mash. I don't salt.) and leave to simmer for about 10 minutes until the outer edge is softish. A fork needs to be able to easily cut them. (If it stays smooth leave for a bit longer).
3. Roll with it. The secret to good roasties is HOT oil / fat / lard. Using a long handled spoon lower each spud into your hot oil (beware hot oil splatters). Roll your potatoes in hot oil or goose fat until they’re completely covered. Reserve the fats from whatever joint you're cooking. About half / two thirds way cooking, drain the fats off into a container. Dunk the spuds in the fats, if there isn't enough to dunk, give them a shake, make sure they're covered all over. Put into a separate oven tray to roast.
4. Take your time. Roast your spuds for at least 40 - 50 minutes.
5. Give 'em a turn. Halfway through the cooking time, turn your spuds so they're evenly cooked. Soggy bums aren’t just a baking bad. I use a fish slice (the thing you use for flipping fried eggs) to turn them.
Think about the timings and the temperature.
When you're cooking a joint and going to be doing roasties, remember that the joint should stand for ten to fifteen minutes after it comes out of the oven. The roasties can continue to cook at a higher temperature, to finish them during this time. Serve them straight out of the oven.