Right tagwag ,pay attention-open your hand as if to shake someones hand,stick your thumb up and put your mouth in the soft skin where thumb joins fingers.Suck ,making sure you create a seal with your lips.
Practice this before getting to the fields as a bad squeal can be just as disastrous as a gun shot in the wrong direction.To be suck-ssesful you will need a good range of calls all done on the hand.Generally speaking ,if a fox is a long way away then a good loud call should get him on.As he approaches ,soften the call as if whatevers making the squeal was moving away from charlies approach