To get started, you need some hollow material like bamboo, knotweed, reeds, hollow bone or pithy wood branches that you can ramrod clean. Then you'll need a sharp knife, and it doesn't hurt if the blade has a serrated section to handle some of the trickier cuts. Cut a section of whistle body about 6 to 10 inches long and less than 1 inch in diameter. The skinnier the whistle is, the sharper and higher the noise will be. It's ideal if the tube is closed at one end, such as a node in bamboo or knotweed. If it's completely hollow like a piece of pipe, then you'll have to plug one end completely.