Press the perineum with the heel of one foot; place the other foot on top [heels stacked]. Having done this, rest the chin on the chest [and lift the chest to the chin]. Remaining still and steady, with the senses controlled, gaze steadily into the eyebrow center; it breaks open the door to liberation. This is called siddhasana [adept's posture].
This is a specific way of sitting cross-legged that encourages all three bandhas (energetic gates) by reminding us to engage the pelvic floor (where the heel touches), the lower abdominals (to sit tall), and the upper back (lifting the chest, shoulders back). The eyes rest on a drishti (gazing point) so the senses turn inwards.
Once the hips are flexible enough, the legs will rest on the floor and you can tuck your toes between each calf and foot to further stabilize. If your legs are not yet resting on the floor when you sit like this, put a block or folded blankets under your sitting bones to elevate your pelvis until your legs can slope downward from your hip sockets toward the floor. If this still leaves the legs tensed, then keep the bottom heel where it is, but lift the top heel and put in in front of the bottom one, instead of on top of it. If the eyes are straining when you direct them toward the eyebrows, then rest them just past the tip of your nose.
Just as moderate diet is the most important of the yamas, and nonviolence, of the niyamas, so the siddhas know that siddhasana is the most important of the asanas.
Siddhasana directs the body to be stable and open, the senses to turn inward. Now you can focus on your breath and how things ARE.
At the beginning of your practice, start your ujjayi breath here and sink your attention into it. Then form your intention and begin to move through the other asanas.
At the end of your practice, let go of your ujjayi here and just watch the breath as it is, taking in all that is in this moment.