3 medium-large potatoes
1/3 c. vegetable oil
Turn the oven on to 425F. Find a cookie sheet with a rim. Don't use a completely flat one or you might set your oven on fire. You need an edge to keep the oil in. Scrub the potatoes. Cut them in half the long way. If you want thinner fries, chop them in thirds the long way, then in strips. I assume you can figure out how to do strips.
Pour the oil on the cookie sheet, put the potatoes in the oil, and mix everything around with your hands so the fries are oily all over. When the oven's the right temperature, stick in the sheet. Cook for thirty minutes, then stir them around with a spatula, prying up the ones stuck to the bottom. Stick back in for twenty minutes.
Look in at this point. If they look nice and brown and edible, take them out, salt them, and eat them. If you want them slightly crisper, stir them around, and stick back in for another ten minutes. Set a timer. Don't assume you'll remember.