How to roast potatoes in the oven to make them crispy?

First you want the right potatoes, a medium starch content works best. Waxy potatoes don't work, they have too much water in them. Maris Piper potatoes work well, in the US Yukon Golds are a good choice.

Next you have to peel them, don't try and get them crispy with the skins on. Crispy skins are great, but you'd use a different method then this to get them.

Boil them in salted water until you can get a fork into them but they aren't completely done. Drain them while still in the pot and then shake them around a bit as it will rough up the outsides, giving a good surface area. This is the real secret, the rough outside of the potatoes.

While the water was heating up you needed to be putting a baking tray in the hottest oven you can manage or just below the smoke point of your oil or fat. Do not use olive oil, butter or anything with a low smoke point. Goose fat, duck fat, peanut oil, canola, sunflower will all work, it all depends on how thick you want your arteries to get ;) . 200C (about 400F) is a good temperature to shoot for with most oils.

Once the potatoes are boiled, drained and roughed up get the hot pan with the oil out and pour the potatoes in, spreading them out. Tilt the pan so the oil pool at one end and use a big spoon to coat the potatoes in the oil, then get them right into the oven. Really crispy potatoes take about 45 minutes, it helps if you take the pan out halfway and turn the potatoes over, then do the tilt and spoon trick again. Once they're done get them out of the oil into a bowl, you don't want them sitting in the oil when you serve.

If you overboil the potatoes you can come back from it, you simply shape the loose potato with your hands and drop them onto the pan, the outside will harden and get crispy, holding it all together. I know people who boil their potatoes until they fall apart on purpose as they like them that way, to me it's too much extra work, but they do get really crispy.

Boil the potatoes first until soft , drain water put lid on the pot and give the potatoes a shake , heat some oil and butter 50 / 50 mix in a microwave until butter has melted cover the potatoes with it season with salt place in hot oven till crispy .

You will want to boil them first, starting in cold water. When they are well cooked but before they crumble into mash, get them out gently. Then I'd set my oven to about 200/210 degrees Celsius. Coat your potatoes with the fat of your choice. Goose fat is great but you can use veg oil too. Don't use oils that can't sustain high heats though. I'd add some rosemary and thyme too. And salt.

Then give it around 20 minutes, if they're golden turn them over and give them another 5 to 10 minutes. Keep check every 5 minutes after that and take them out when they've reached a nice golden colour.

The others cover the boiling method, but I want to point to something you are doing wrong in your current method. Namely venting your oven. Your oven should have a vent to release excess humidity. By opening the oven so frequently, which you also do when turning, you are allowing the heat from the oven to escape. For a good crisp potato you need high heat and you need to maintain that heat. You aren't doing that. A crispy dry should be turned only once during cooking approx halfway through.

Coat your potatoes in a seasoned corn starch blend, then coat them with an oil/fat, and finally roast until done.

You're welcome. :)