You don't need to the soak the potatoes in cold water. Soaking them in cold water removes some of the potato starch and makes them crispier. But the air fryer does such a good job of crisping these up that soaking isn't necessary. On the other hand if you want the crispiest potatoes possible you can soak them. In that case put the diced potatoes in a bowl of cold water for 30 minutes to overnight and then dry them off and proceed with the recipe.