Remove any giblets from the turkey. Rinse the cavity and the outside of the turkey with cold water, then pat it dry. Set aside.
Measure the correct amount of kosher salt based on the turkey's weight. Blend the salt and rosemary leaves together using a blender or mortar and pestle.
Sprinkle a little of the salt mixture into the cavity. Then, turn the turkey onto its left side and sprinkle the right side with 2 to 3 teaspoons of salt, adding the most to the thigh. Turn the turkey over and repeat with the other side. Sprinkle the remaining salt over the turkey breasts.
Place the turkey into a large zip-top bag or into a pan and cover with plastic wrap. Place the turkey into the fridge on its back and refrigerate for three days, turning the turkey over onto its breasts on the last day (optional).
Dry the turkey by placing it breast-side-up on a plate in the refrigerator the day you plan on roasting it. Refrigerate uncovered for 6 to 8 hours.