How can I mate a stair skirt flush with the drywall below?
There are 3 options to fill the gap:


Assuming you want to fill the gap with new drywall of even width you first want to make the cut even. So find the widest section and then open a compass to that measurement. Attach a pencil and run one edge against the top edge/wood and the bottom pencil will mark a parallel line on the drywall. Use a utility knife to score and cut the drywall along the line (oscillating saw can also work, easier but dustier).
Cut a piece of drywall the width of the compass and place in the gap and screw into the currently exposed wood and fill small gaps with plaster/putty.
Or if you don't want to take more away and you are fine with using more putty measure the minimum width and cut some drywall. Position in place and cover leftover gaps with putty.
To get an exact match, but requiring extra work and lots of time/precision/frustration, you can lay painters tap on the top piece flush to the edge. Then open a compass to the widest of the gap and trace the edge on the tape (the other following the drywall edge). Once done, comes the tricky part, transfer the tape to the drywall. This can by done by measuring the bottom gap width, measuring from the edge of the drywall, over the tap to the bottom marking and taping the portion down. Then using a t-square press the rest of the tape down (or measure from the edge of the drywall to the tape and make sure that it doesn't change).
Cut along the line and attach. It will be a really tight fit and may need additional trimming. Putty any gaps, of which there shouldn't be many.


THEN:

Sand and paint.