What does it feel like to be very relaxed and very stable? How do we bring it about on a repeatable basis so we can go there almost at will and not take forever in the process?
First attend to the external factors. Are we warm enough? Are the clothes we're wearing loose, comfortable and not restricting us in any way? Have we opened up this small block of time free of interruption (did I put the cat out?). Are we hungry or thirsty? What other things might get in the way for you? Take a moment to notice what they are. Make a note to come back to them after meditating. Often really important tasks and deadlines I have forgotten about will pop into my head as soon as I quiet down. After all of these external factors have been taken care of, you can turn your focus to your posture and balance.
The saints and sages sought to effectively exit their bodies and go into the realm of consciousness so completely that the physical self was left behind to care for itself in many ways. The postures they adopted rely heavily on triangles for the stability they needed. Sitting cross-legged creates several triangles that connect to form a tetrahedron.
This is easy to see if you sit with your hands resting on your knees. Lateral triangles magically appear from the shoulders down the arms to the knees and back along the thighs to the upright torso, when you do this. Any tendency to collapse to the side is arrested, as is falling forward. If you sit towards the front edge of a cushion with one foot drawn in close to your body and the other placed on top of it or in front, then your knees and calves are touching the floor and forming a stable base.
Your head and torso are in now an effortless neutral. Leaning very slightly forward before lengthening upward is one way to find this. Let your breathing take you there. When your breath is free to flow fully in and out as your chest and torso rise and fall with each cycle. You have taken your seat... so simple.
In the end all this is about supporting the breath. Finding dynamic stillness where we inhabit our physical body so it that it seems to disappear and become a minimal focus. Then we are free to open to primary awareness and explore other fields of consciousness.