Make pencil marks on the vertical studs of your wall every 16 inches, starting from the floor and working upward.
Measure the width of the wall. Cut a series of furring strips to match the width. Nail the furring strips horizontally across the vertical studs with 6d finishing nails, using the pencil marks on the vertical studs as a guide. Place a final furring strip at the top edge of the wall.
Measure the height of the wall. Cut your tongue-and-groove boards to match the height of the wall. Starting at one side of the wall, place a board against the wall. Place a level on the edge of the board to ensure that it is plumb. Nail the board to the furring strips with 6d finishing nails.
Place the next board against the first, locking them together with the tongue-and-groove slots and tabs. Nail the board to the furring strips.
Continue placing boards until the wall is covered. Cut the last boards to the proper width if necessary for a proper fit.