From the position for normal breathing, you move your soft palate upwards and shut off the nasal cavity. Then you round your lips, leaving a rather close air passage there, and at the same time, you raise the back part of your tongue pretty high towards the roof of the mouth cavity, but not touching it, thus leaving a rather narrow air passage in the back part of your mouth cavity. You breathe out, letting your vocal cords vibrate.
Wait a minute! You’ve just pronounced a /__/ sound ! Then you lower the tongue, particularly its front part, to a very open position, never stopping the vocal cord vibration. Then, after about 60 msec. or so, you move the front to central part of your tongue towards the position which is just above the close-mid.