I prefer to use cooked potatoes, cooled, box grated. When you make bake potatoes, stick a couple more in for extras for hash browns.
Heat a (preferable) cast iron pan medium hot. If you don't have CI, a non stick pan works good too. Add oil, I like peanut oil, any veggie oil works or bacon grease. When the oil shimmers, add the potatoes, and spread evenly. A thicker layer rather than a really thin layer is better if the potatoes are already cooked. Walk Away. Leave them Alone. Do not hover over the stove. Do Not Touch. After some 4-5 minutes, check under a corner. Once they have browned, do the plate thing and tip them out. Add a little more oil. Invert the potatoes back into the pan and cook the other side. I like to put a couple gashes with the spatula across the top while the 2nd side cooks to release steam. I like hash browns a little drier. The insides should finish so they are almost creamy in flavor or texture. Tip them back out on a paper towel lined plate if there was too much oil, and blot up any excess oil on the top, if there is any.
After while you will learn how many swirls oil to put in the pan so they are "just right".
My Favorite breakfast diner chef runs a swirl of oil across the tops of his potatoes after he lays them on the flat top, then a 2nd swirl of oil while he holds the taters up using a big spatula when flipping them. I tried for years to make good hash browns. So I tried to strategically sit at the counter when possible and watch him work. I don't do the oil topper at home on the un-cooked side. I don't think it's necessary. And rarely if ever do I have to blot up extra oil when served at the diner. He's got the touch we all wish we could achieve.