The stiles are the vertical pieces of the shutter frame. The scale of your window may change things, but I prefer to make them 1.25" thick (deep) and 1.5" wide. The length will be determined by the height of your window.
I make these from the 6/4 rough cut lumber. I prefer to buy a piece wide enough that all four stiles can be cut from it. This is a picture of the pieces put back together after I cut them:
Now that you have four distinct pieces of wood / future stiles... use the same roundover bit on each edge to give a more professional look.
Next you will need to drill pilot holes for the screws which will hold the rails and stiles together. The bit size will depend on your wood and screw size. I believe I used an 1/8" bit with 2.5" #8 screws. This shows where I drilled the holes:
The hole closest to end is 1" from the end and the other is 2.5" from the end. Each is countersunk and centered within the piece.
Finally, you need to drill holes to receive the louver pin. These should be 1/4" diameter and 3/8" deep. The placement of these holes goes as follows:
If you have 3" louvers and want them to overlap by 1/4", place the holes every 2.75". Center the holes within the stile and make sure you choose the right number. You may need to adjust the rail height to accomodate the louvers.
This is a picture of the holes I marked:
and then drilled: