Measure the width of each wall. Write down the measurements.
Sketch your design to get an idea of how many boxes you want. Boxes can be simple squares or rectangles. For example, you might have two wide rectangles on each wall.
Calculate the size of each box. Start with the wall measurement in inches and subtract the width of each side of each box. For example, if your wall is 100 inches wide and you're building four boxes with 2-inch molding, you'd subtract 8 inches from 100. Divide the remaining number by the amount of boxes to determine how much space you can use for each box. With this example you would divide 92 inches by 4, which will give you 23 inches per box. If you want the boxes to be 20 inches wide you will have 1 1/2 inches on each side. That will leave 1 1/2 inches between the corner of the wall and the first square, and 3 inches between each square.
Cut the molding to your desired length with a small saw. Each end should be cut at a 45 degree angle so the top edge of the molding is shorter than the bottom. The corners will come together when you attach all four pieces to make the box. Make even 45 degree cuts on each piece.
Sand the edges of the molding.
Paint the molding and the walls. You can use the same color for both, or choose contrasting colors to make the wainscoting stand out more. For example, paint the walls blue or red and use white molding and trim.
Mark the walls with a pencil so you know where to install the molding. Use a long ruler or yard stick to draw straight lines. Use tape or a small "X" to note where the studs are.
Attach the top piece of molding in your first square. Use a nail gun to hold it in place, driving the nails into the studs. Add the sides, then the bottom to complete the square. Repeat steps 7 and 8 until all of your squares are installed.
Install the chair railing or top ledge and the bottom piece to complete the look.
Check your work with a level. Make any adjustments necessary.