To draft the pattern of this gown start from a basic block with darts, drawing the full front for the asymmetrical dart manipulation.
Onto both front and back draw the panels flaring them from the knee down and draw the shoulder strap a couple of cm inwards from the shoulder edge and 3-4cm wide.
Onto the back, draw also the neckline and distribute the dart shaping also from the center.
Onto the front, draw the neckline and widen the shoulder dart by 1cm at it, then shape the underbust and draw the slanted line for the dart manipulation.
On the skirt side where the slit is, do not flare.