Prepare the branch by finding a section with consistent thickness. Trim the branch to 8 or 10 inches. Cut one end so it is at an angle.
Using the knife, cut a small hole in the top of the branch. This can be done by pushing the knife into the branch slowly. The result should be a hole the size of a pen head.
Remove any bark from the branch, but keep it one piece. This may reduce the overall size of the whistle to about 6 inches.
To allow air into the whistle, cut a circle near the tip of the angled edge. The circle should be cut around the bark. Cut a small shaft in the mouthpiece.
Cut a shallow, round trench on the top of the whistle. This trench shouldn't be more than one-fourth of the total depth of the branch.
Wet the branch and reapply the bark so the sound from the whistle is reduced.