Spinal Curves from Bottom to Top
When you are seated with a neutral pelvis, your lower back will have a natural sway. But be careful you are not trying to mimic the curves of the lower back when you are sitting upright by lifting the chest and rotating the ribcage backwards. You'll know you are doing this if the lower, front ribs thrust forward. This "rib thrusting" actually leads to a neck and head that are in a forward position relationship to the ribs and actually leads to too much curvature in the upper spine. It also tends to compress the lower back more rather than solving the issues with rounding the lower back.
The ribcage is meant to be vertically stacked over the pelvis, so for most of us, that means allowing the front lower ribs to drop down and shift backward.
Then, work on ramping the head to restore the natural curve of the neck.
To summarize:
start with a vertical pelvis where you regain the sway in the lower back
drop the ribs to stack the ribcage over the pelvis
then ramp the head.
