Union of Prana and Apana by Sivananda Saraswati.

Union of Prana and Apana by Sivananda Saraswati:

Sit in siddhasana or Siddha yoni asana. Relax the whole body for a few minutes, bringing it to the point of absolute immobility. Now become aware of the natural abdominal breath. 

Center your awareness on the movement of the navel as you inhale and exhale. Continue for some minutes. Now become aware that there are two forces traveling to the navel - prana, and Apana. One force (Apana) is ascending from mooladhara to the navel, while the other (prana), and is descending to the navel from above. They must both reach the navel at the point of full inhalation. When you feel that the two forces are meeting in the navel, perform kumbhaka, retention of breath, and then develop mental awareness of the single central point of force in the navel. Do not strain. Release the breath and continue this practice in your own natural rhythm.
The awareness of the two forces traveling and meeting in the navel center must be simultaneous.
Now, as the two forces are converging in the navel, gradually allow moolabandha to take place. Go on contracting moolabandha as you heighten your awareness of the force which is centered in the navel. Hold your breath for as long as you can, while centralizing the force in the navel and performing moolabandha. As you release the breath, release moolabandha as well. Do not strain. Go on go on practicing for 3 minutes or more. Practice this Kriya for at least 1 month.