The stick you are going to use has to be green and 5 to 6 inches long whereas its diameter should be about half an inch. Furthermore, its bark should be intact. It would be good if the stick you choose does not have any knots or twigs and is fairly straight.
Use a carving knife to cut both ends of the stick in order to make them straight and flat. Mark one of the stick ends with a marker in order to keep track of the starting edge. Make two more marks along the stick; the first mark should be 1 1/2 inches away from the starting edge whereas the second mark should be 2 1/2 inches away.
Again making use of the carving knife, cut a notch near the grease pencil mark closer to the starting edge. The notch you cut should be in the shape of the letter V.
At the second grease pencil mark, cut the bark of the stick deep along the entire perimeter.
At the point where you made the second pencil grease mark, peel away the bark, thus forming a thin ring on the stick. Dip the stick in water at the starting edge all the way up to that ring. Remove the stick from the water and tap on the bark very gently. Continue to do this until you are able to twist the bark in such a way that a bark ring is formed around the stick.
You now need to cut out the whistle cavity. For this, measure 1 1/2 inches along the bare part of the stick and mark that point. Make another mark another 1 1/2 inches farther from the initial mark. Connect the two marks with a line cut all the way to the middle of the stick. You now need to whittle the wood falling in between the two marks.
From the starting edge all the way to the cavity, you will need to create an air passage. This can be accomplished by whittling the length of the stick falling between the starting edge and the cavity you created in the previous step until the entire area has been lowered properly.
Complete the task by sliding the bark back into its original position on the stick.