Posture as such cannot be corrected. The character of a posture can be changed. The character of a posture is created by the person’s manner of use - the way the body parts are held in relation to one another, and the muscular arrangement in which the whole self musters force for action. Sitting is a posture. The way you sit - whether in a collapsed over-relaxed shape; or in a tightened down shape; or in an opened out, lengthened and widened shape; or in an over-lengthened and narrowed and stiffened shape - gives the character of your sitting posture.
As two- year-old Roger Chen investigates his environment he adopts several postures. He reaches up, leans forward and around , reaches out, turns to look at Mum, holds his toy in his lap - and in each posture the lengthened out shape of his trunk is maintained as a constant. The parts of his body not required for a particular action remain free - his legs while he’s sitting, his arms while his mother holds the bottle for him to suck. His legs don’t need to be crossed over one another to help him stay balanced in sitting; his arms are not being held tensely while he’s not using them. His overall shape is kept open throughout the flow of movements, so that in whatever he does the safety and well-being of his whole body is being taken care of. His manner of use is good, giving good shapes or what we perceive as "good posture".