First of all, pick up a duck call and blow it to take a good idea about the sound. Do experiments to determine that which sound it will be easier for you to produce. It is better to do it in front of ducks to see their response.
Once you have taken a good idea about the different sounds, it is the time to create the same sound with your hands. Hold your left hand with the right one, with both palms facing each other.
The right thump should be towards your left wrist, whereas the left thump must hold the index finger of your right hand. You are actually required to create a small hole between your palms, so that you can use it for blowing the duck call.
It is the time to blow the call. Speaking ‘quack’ is one of the basic duck calls. Make sure you are crisping the ‘ck’ part of the word. Pronounce this words in different sounds until you get the desired sound.
When blowing the duck call, try to speak different words like "kwit" and "dwit". Once you succeed in making these sounds, you need o play with the pitch of the sound. Position your hands in different positions to vary the pitch.