It is very important to commence your meditation practice half an hour before sunrise and sunset and this time is called Sandhi kala. During this time dual nostril ( Sushumna) operation is in progress and aids in centering. Another important point to note is to expressly state the purpose and time for which you are going to meditate. You can choose any asana (posture) with a relaxed body and vertical spine. You could sit on the edge of your bed or a chair with your back away from the backrest.
The next important factor is the position of your eyes. You can choose Shambhavi Mudra , where you focus your closed eyes on the middle of the eyebrows ( eyebrows center) and be in a relaxed state or you could choose the Agochari Mudra , where you focus your eyes on the tip of the nose near the nostrils. You could use Chin Mudra of the hands when you are seated on the chair so that the back of the palms rest on the thighs. If the place is noisy, you may plug your ears with earplugs.
Once you have relaxed in your asana with the above mudras, it is time to start your meditation. Imagine Mahaprana (Vital Cosmic energy) all around you and a hollow body within. You are now absorbing this Mahaprana from the space around you ( This space and Prana transcend the entire Cosmos). This Mahaprana is called prana when it enters your body. Prana being energy is not perceptible to your senses. Now, clearly feel the path of the pranic energy ( not the physical air youbreathe) as it enters your body. The starting point of this prana, outside the body is called BahyaDvaDashanta (BDD) ( external point twelve finger widths from the eyebrow center). This prana then enters the nostrils, goes to the eyebrow center , to the top of the head and rests at a point twelve finger widths below the throat center , inside your body. This point is called Hrit.
So, the inhalation goes from the external BDD to the Hrit point. Now, with the eyes in Shambhavi Mudra, listen to the sound of the breath as it moves in and out. Be with the breath as it moves from the outside-in to inside-out completing its cycles as you visualize the path in your mind’s screen. If you continue this process for half an hour, you will achieve centering. If you feel drowsy, you may lie flat on your back on the bed and the practice will continue. If instructed by a Guru, you may merge a mantra like “Om” with each inhale and exhale or you could merge the HamSah (हंसः) mantra which is being chanted automatically without your awareness. Use Ha (ह) with the inhale ending with am at the Hrit ( Ha + am ० = Ham (हं) and Sa (स) with the exhale ending with ‘ah’ at the BDD ( Sa + ah := Sah (सः)