More in Windows Walls and Doors
Use a table saw to rip the boards down and the chop saw to cut them to appropriate lengths.
Make frames for the wall extension. One to extend the full height of the wall, and another shorter counter-height frame right next to it.
Nail the tall frame to the existing wall using 3-1/2" nails and the impulse framing nailer. Anchor into the floor and ceiling. Then attach the small frame the same way. Once both walls are straight, nail the base plates into place.
Cut 1/2" drywall to cover the new extension, and use 1-1/2" drywall screws to attach it. Make sure to mark and cut out holes for the outlets and switches.
Note: Have an electrician run wires into the new wall for outlets and switches.