The shutters are for more than just looks. They actually secure all of the little pieces of siding that are left when cutting out the windows.
each shutter is made from six individual pieces. Four that are 24" long and Two that are 8" long. Again, stain all sides of these pieces prior to installing.
Clamp the first long piece along the edge of the window ensuring it will overlap top and bottom onto full length siding slats. Now, go inside. You need to pre-drill all of the small slats since they WILL split if you don't. Then just secure the first part of the shutter from the inside through each slat.
Now, the second long piece of shutter is just for looks, and it is usually over one of the stringers so you won't be able to screw it on from the inside. Just secure the piece with two screws. Pre-drill them and locate them under where the shutter cross pieces will go.
Then pre-drill and screw in place the cross pieces from the front.