I teach this regularly in my meditation and qi gong classes. We build good posture from the bottom up.
The most important basic elements of sitting are:
Hips higher than knees, so the thigh bones are angled down.
This allows the sacrum to be vertical, like the head of a spear stuck upright in the earth.
This allows the lower vertebra (below the ribs, behind the belly) to be straight up and solid, like the shaft of a spear.
The vertebra of the chest (ribs) rise in a gentle curve. Above that, the next rises in another gentle curve.
The shoulders are spread wide, neither hunched, nor pushed back.
Every few minutes of work, we return to a symmetrical posture, close the mouth, breathe through the nose, half-close the eyes, and restore symmetry and balance.
A low desk or box right in front of you for close-up work, even if more is spread around you, is a good idea.
the legs can be in any comfortable posture - either cross-legged, or, if you are flexible enough or get a seiza bench, tucked under you.