The basic principle is that you should look for a posture that is relaxed and alert, echoing an optimum mental state. This can be achieved by having a good ‘triangular base’ between your seat and your knees. In the cross legged position the knees should be on the ground, or if not, they should supported by cushions.
The height of the seat should be such that your back is neither arched, because the seat is too high, nor slumped forward because the seat is too low. (The Kindseat allows you to set this easily) A tip is to check that the sitting bones are pointing downwards. (Not tilting forward or back)
From this stable base find a back position that is neither leaning forward or back. You could try rocking forward and backwards (less and less) to find a point of balance.
With the chest lifted a little, allowing your shoulders to move back and down, move your neck and head until you find a point of balance .
Check that your posture is at ease, your face soft. Your eyes can be half open or gently closed.
It’s a good idea to have your hands supported in front of you. Perhaps resting them on a cushion on your lap. This helps your shoulders to relax.
For more guidance have a look at The Wildmind Meditation Posture Workshop