Use a miter box to cut corner blocks from 2x4 material. The short sides of the triangular blocks should be 2-1/2 inches long.
Stand the 1x6 on an edge. Place the 1x4 on top and align edges flush. Fasten the pieces together using 6d finish nails.
Place the corner blocks at 12-inch intervals. Fasten the blocks using 6d nails. Nail into both the 1x4 and 1x6. Be sure nail points don't protrude.
Locate wall studs and fasten the plate-rail frame to the wall by driving nails through the 1x6 into each stud. Use nails long enough to penetrate 1-1/2 inches into studs.
Cut decorative moldings to fit between the corner blocks. Fasten the molding to the frame using 6d finish nails. For best results attaching small moldings, use a glue gun and hot glue. Apply glue to the back of the molding and set it in place. Hold firmly until the glue sets, about 30 seconds. Clean up excess glue immediately.
Apply corner molding around front and sides of the top of the plate rail. Cut corners of the molding using the miter box.
The many stock moldings available today in lumber stores and home centers beg for imaginative uses. By combining different moldings, it's possible to create custom decorative details that are uniquely fitted to your home and tastes.
This image shows a basic plate rail -- a piece of 1x4 for the top and a length of 1x6 for the back -- embellished with a simple style of crown molding.
A piece of crown molding can also work well used as a base cap, the term for a decorative detail affixed to the bottom of the 1x6.
Another option is to add a narrow quarter round piece along the front top edge of the rail.
Here, a piece of casing has been combined with a piece of crown molding for yet another different look.