Finishing off bottom side of sill plate with drywall on wall raised above floor

[JMDbeginner]

I am trying to finish off the underside of a wall (with drywall) that is raised above the floor adjacent to my stairs.  Also, how can I attach the wall to the full vertical one without the corner changing? Any help is appreciated.  Thanks.

[scottharris]

Can you post your plan?

[JMDbeginner]

Sorry for the delay.  Here's the plan.