This allows the spiritual energy to flow freely up the spine, which is an important aspect of meditation. Leaning against a chair back, or a wall is perfectly all right. If, for physical reasons, you cannot sit up, lay flat on your back. Place your hands in any position that would be comfortable. If it does not go against your beliefs, call on a higher source for assistance in your meditation.
Any form is all right.
This can be quite helpful, but is not necessary. Instructions for relaxation meditation This remarkably easy and relaxing meditation makes use of a little known secret about the eyes. Allowing the eyes to rest in a soft downward gaze has an instant, automatic relaxing effect. Sit comfortably with your spine reasonably straight. Allow your eyes to rest comfortably downward, gazing softly, but not focused on anything.