Okay, so once you have everything you need; put the pan on the stove and turn it to medium. When you think it's warm enough put all of your chopped/diced goods in the pan. As you mix it around, pour in the olive oil and continue mixing.
After a few seconds put in your celery salt and rosemary. Usually, it will take 2 to 3 minutes. Then... TADA! Mushroom surprise :)