Go out and find yourself a fine elder.
Cut a piece of elderwood, 1,5cm thick, about 20cm long. Also cut another piece of wood. It's not important what kind of wood as long as it is hard.
Get rid of the soft inside of the elderwood, except for about 5cm at the end. So your whistle isn't fully open.
Cut a V shaped form in your whistle. 3cm from the front.
Cut the other wood as shown on this picture. Make it exactly as thick so it fits nicely the inside of your whistle.
Put the piece you just cut in the front of your whistle, it's ok if you can see it appear through the v shaped hole.
Optional: make a small hole in the bottom of your whistle so you can change the tone.