The easiest way to describe. Put the call in the roof of your mouth. Using your tongue put pressure on the reed and blow. The amount of pressure depends on the call. You want to produce one high clear note. Keep practicing that until you can produce and hold that note.
When you have mastered that , while you are making the high note, reduce the pressure on the reed and you will hear it break. Keep practicing that,
When you have mastered that speed up the tempo and it becomes a yelp.