Adrian's got some of the best people to Turkey translation I've ever seen given, I'll add this, it may help.

The first step is to understand the consept of the tape blocking the airway. Now apply tongue pressure and just get it to squeek at a high pitch, (''EEEEE") then by either lowering the jaw or just the tongue, lower the pitch (OOOOP)

This may sound strange, but I first associated a Yelp with a donkey type sound (EEEOOORR) as far as how to shape or vocalize your mouth.
Take Adrians vocalization and think of "POPEYE'S (yes the cartoon sailorman) laugh, YUK,KUK KUK KUK KUK

I wont say they're the best, but MAD seam the easiest. After you get it down they seam to squeek to easy, and stay away from cutters to start. I don't care for plastic framed calls either. A simple 2 reed no cut or inverted V

Hope this is helpful, Good Luck