Placket

When attaching the placket to the front of the shirt, where does the collar go? If i follow the directions it looks like the collar will be turned to the inside when the placket is cut down the middle and folded to the inside. Also why do I need to sew along the back of the collar since that part is already sewn and the placket doesn't attach there? I am pretty sure I am doing something wrong here and would appreciate some help. Thanks.