There is no specific posture for this meditation. There is really only one thing that matters—sit comfortably. For some people, sitting on the floor or on a cushion cross-legged is comfortable. For others, sitting in a chair is more favorable. After awhile if sitting upright becomes an effort, then we slouch. If slouching becomes an effort, then we slump. It that becomes an effort, we then lie down and continue to meditate.
It is important, however, to begin each meditation session sitting upright. This position tells the body that it is getting ready to meditate so the body can automatically respond. It sets up a certain brain wave function in the physiology that is conducive to transcending.