Sit in a comfortable position with your back straight. This might be on the floor on a yoga mat with your legs crossed or on a straight-backed chair. Close your eyes and begin taking slow, deep and long breaths in through your nose and out through a slightly opened mouth. Take several breaths and allow yourself to relax with each one.
Close your epiglottis partially, contract the back of your throat and continue to inhale through your nose and exhale through your mouth. As you inhale, feel the sensation of air swirling at the back of your throat and try to create a deep, full resonant sound -- an audible soft hissing or drawn-out "ha." Exhale slowly through your slightly opened mouth creating the same sound. Take several breaths in this manner until the sound is continuous and smooth.
Practice generating this same sound with your mouth closed. Constrict your throat, inhale and make the soft hissing noise. As you exhale through your nose, allow the air to swirl at the back of your throat and make the same "ha" sound you made when your mouth was open. The sound should come from your throat and not your nose. Stay relaxed and avoid forcing the sound. It doesn't have to be loud, just enough so that someone close to you can hear it.
Count to yourself as you practice to lengthen the inhales and exhales. Start with five-minute sessions and count to four as you inhale and four as you exhale. Work your way up to 15-minute sessions with a count of 20 for each inhale and exhale.