Gently rinse mushrooms and dry between paper towels. Cut into slices. Place on a cookie sheet lined with heavy-duty aluminum foil sprayed lightly with cooking spray. Put into a 200 degrees F oven with the door slightly ajar. Turn over with a long handled pancake turner every 15 to 20 minutes. Depending upon how wet they are, it should take a couple of hours for them to dry. Remove the cookie sheet from the oven and place it on a kitchen counter for a few days. When the herbs are absolutely one dry, transfer them to a Mason jar with a tight fitting cover. Make sure they are absolutely brittle or they will mold. Label them.