First of all, you should take measurement of the wall and if your wall has a standard size then mark it between 40 to 46 inches upwards from the bottom.
Make sure that you leave at least one inch space beneath the mark and it should be where you want to build the wainscoting.
The mark should be levelled and should also be extended from one corner to the other of a wall where you want to build wainscot.
You should cover wall with wallpapers or paint the wall with colours of your choice. When the wallpaper or paint dries up, draw a horizontal line by leaving one inch space from the bottom for wainscot molding.
Now you need to use an electronic stud sensor for finding and marking each stud. Make sure you have put a light pencil mark below the bottom line. Now you should measure the stock molding with a saw in a way that sits below the horizontal line that you drew.
Then keep marking all the points where molding crosses the marks of the studs. You can also use your trim nailer for sinking a nail at the marked stud points.
Now, it is fixed and you should cut the both ends of the molding from the corners of the wall. Cutting should be straight which will give a perfect look and will not destroy the shape of the molding from the corners. Install the molding at the bottom of the wall in the similar way.