The best time to practice going quiet -- to detect and drop dark inner voices -- is when the world around you is more or less already in a natural state of silence. So, early morning, upon arising, and just before you go to sleep, are the most likely times to yield the best results. But, as you'll no doubt come to discover for yourself, any time is the right time to go quiet.
Find a place to sit where your back can be more or less supported and held straight. Let your hands rest, open or closed, in any position that won't cause tension to themselves, your arms, or shoulders, as you remain seated for the duration of your practice. Twenty to thirty minutes, twice a day, is a suggested minimum time to sit quietly. But do the best you can. There are no laws that govern inner silence. Besides, the day may come when you'd like to sit for longer durations, so you be the judge. Let the length of this time for inner quiet be whatever it wants to be.
Allow your legs to assume whatever position is most naturally relaxed for them. It's better if you don't cross your legs one over the other, as this posture interferes with your circulation, and the ensuing discomfort will become a distraction.
Once your body is situated and in relative ease, close your eyes and let your awareness sweep over the whole of your body. Adjust your limbs again, if necessary, so that no individual part of your physical self is calling out for your attention.
Now, with your eyes still gently closed, let your shoulders take the full weight of your head. You should actually be able to feel the physical transfer of this weight take place.
Then, give the weight of your shoulders and your arms to the armrests of your chair, or to whatever part of your body is beneath them. If you're doing this properly, you'll be surprised how much of your own bodily weight you were unnecessarily supporting without knowing it!
Finally, give all of this collective weight -- head, shoulders, arms, upper body, buttocks and legs -- to the chair or sofa you're sitting upon. Consciously transfer the weight. Let it go. Then let yourself sink into the feeling that comes with releasing all this unconscious physical stress and tension.
The next step is to continue expanding this relaxed and increased awareness of your body to include within it the awareness of your thoughts and feelings. In other words, bring into your enhanced physical awareness the further awareness of what your mind and emotions are doing in the moment. You watch yourself. You're to be an impartial witness to the life of your own thoughts and feelings. Let them fly and hop around within you without the slightest concern for their direction or character. Neither resist, nor let yourself be drawn into any of their attention-stealing antics.
Again, all you want to do is watch. Detached self-observation is your aim. So each time it comes to you to realize that you're no longer watching, but rather that you've been captured by a thought or feeling, and are being carried along by it, just quietly withdraw yourself from that temporary psychic wave. Come back to the awareness of yourself in the Present Moment. This part of your practice is the heart and soul of going -- and knowing -- quiet. You must experience it for yourself. So, as you sit: Let go; give up; go within, and watch. And over and over again, bring your awareness of yourself back into the awareness of the Present Moment.
Pay no attention to what your own thoughts and feelings are trying to tell you the whole time you're sitting. Which is namely this: "You should give up this worthless, unproductive practice!"
Learn to watch, and drop, these dark inner voices. They don't want you to succeed and there's a good reason why: They can not dwell in that silent world you wish to enter, and that wishes to enter you.
So persist! You will prevail. For even as you struggle to stay aware of yourself in the Present Moment, that moment itself, changes. And as it does, so do you.
Slowly, subtly at first, but eventually even beyond those protestations raised by your own mind, the distinction between your sense of self and your awareness of the Present Moment melts away. And as it does, a new, deeper sense of silence floods into you; filling your awareness with itself and, at the same time, with yet another Awareness that the source of this supreme stillness is arising out of your own Being. It washes everything out of its way. And so arrives a quiet mind.