Cut your planks into 3” strips. Use your miter box and miter each end that touches the wall to a 45 degree so it will sit flush with the wall (if needed) Level and nail in the first plank right above your fireplace.
Use a few quarters or nickels as spacers and cut and nail up all your boards. If you have a light switch, measure accordingly and cut out a space for the light switch using your jigsaw.
When you get to the bottom, you may need to cut back your baseboard. Simply use your multi tool and cut it off.
Measure your sides and cut 2 long pieces to use as trim pieces. Using a table saw, miter one side of each trim piece at a 45 degree so it will sit flush to the wall (if needed). Pin nail them over the existing planks.
Putty, sand and caulk (the sides of the trim next to wall if needed) and paint.