The ideal is an upright, alert posture. Slumping only increases the pressure on the legs and discomfort in the back. It is important to attend to your posture with wisdom, not insensitive will-power! Posture will improve in time, but you need to work with the body, not use force against.
This is not a comprehensive or exclusive guide, but a suggested outline for practice.
Meditations are strongly recommended to seek a trustworthy and experienced 'spiritual friend' or teacher for ongoing advice.
May all beings be at peace;
may all beings be free from suffering