Start by saying the sound 'ah' as in father and then hold your open hand out in front of your face with the palm facing the floor -- in other words parallel with the floor. You will be looking at the profile of your index finger and your thumb. Now, while saying the sound 'ah' slowly move your hand towards your throat, above the Adam's Apple or below where the chin meets the neck. I imagine most who are reading this know how the 3een should sound like so, that said, when your hand reaches your throat keep pushing (slowly) until it sounds like you think it should. I looked at my profile in the mirror while doing this to try to judge how far I push my hand into my throat, but it is difficult to tell -- maybe anywhere from a half inch to an inch.
Anyway, this is a good exercise just to get you familiar with producing the sound, the muscles that produce it, and what they need to do to produce it. Eventually, with enough practice, one should be able to produce the sound without choking him/herself.
Using this technique try saying a few words, such as 3een (eye), 3ilba (box) or tin, 3ilaaqa (relation), or whatever. Produce the sound using the technique and then right before saying the rest of the word pull your hand away really fast so you can say the rest of the word.
