The rails are the horizontal components of the shutter frame. I make them from 4/4 lumber.
The hardest part is determining how tall the rails should be. I use the rail as a buffer to absorb extra space or to allow more room when needed. This is because the louvers will not exactly fill the space. A little experimentation should find a good height. For the proper proportions, I like to have them somewhere between 4 and 5 inches. The length of the rails is determined by the width of your window.
I have found that tapering one edge lengthwise of the rail helps the louvers close more tightly and thus block out more light. The side that should be tapered is the side that will contact the louver. For the bottom rails, this will be the top edge. For the top rails, it will be the bottom edge. I prefer to taper both the front and back of the edge so there is a point in the center. I cut the tapers at a 25 degree angle on the table saw.
You will not need to round the edges of these pieces, so the final step is to drill pilot holes that will match up with pilot holes in your stiles. Use the same spacing and bit size.
This is a picture of my rails before I cut the tapers: