Double-frying the potatoes ensures they get cooked through and stay tender on the inside during the first fry and crisp and browned on the outside in the second fry.
For crinkle-cut fries, simply slice potatoes vertically with a crinkle cutter, also called a wavy chopper or serrated slicer. For waffle fries, after making one vertical slice, rotate the potato 90°, and make another vertical slice. You can find many styles of crinkle-cutting tools on
amazon.com for less than $10.