Determine how high up the wall you want the bead board to reach. Use a level to mark a straight line across the wall.
Disconnect supply lines and drains from the sink. Remove the vanity by cutting through the caulk with a knife and prying the unit away from the wall using a hammer and pry bar.
Measure the wall dimensions and mark the measurements on your bead board. Cut out the panel with a circular saw.
Align the panel with the wall and mark the placement of the drain and supply lines on the backside. Place the beadboard face down on your work surface and bore holes large enough to accommodate the pipes and tubing for the sink.
Apply construction adhesive to the back of the bead board and attach to the wall. Enlist a friend to pull the drain and lines through the holes and line up the panel with the line you marked on the wall in the first step.
Reinforce the bead board and prevent curling or warping by brad nailing around the edges every 12 inches to 18 inches.
Measure the distance across the top of the bead board panel and cut a section of chair rail to that length with a miter saw. Mount the rail on the bead board and attach to the wall with the brad nailer.
Apply caulk along the edges of the chair rail to seal off areas where bugs could enter or moisture could settle. Fill exposed nail holes with caulk as well. Allow to dry and sand off any excess protruding caulk. Add a coat of moisture-resistant paint, if desired.