Huff the air like was stated earlier.The aircomes from the diaphragm not the mouth.
Open and close your mouth with each sound. That keeps the yelps and cuts from running together. For loud clucks try building up a little air in your mouth then pop it out with some force. Forsoft clucks try a little less air and a little less force.
Tongue pressure and tongue placement is key. Try moving your tongue back and forth on the reeds till you find that spot that gives you the sound you want. Also try more or less pressure with the tongue. It will take some time to get it figured out. Good luck.