Turn the last stud on the first wall sideways and overlap it about halfway past the connecting wall to create drywall backing ...

1/2-in. plywood with 5/8-in. particleboard underlayment. Remove the particleboard and screw the plywood to the floor joists with 1-5/8 in. drywall screws spaced every 6 in. Install a layer of 5/8 ...