You may lay the bag on the anti-static mat to place the bag at the same potential as the mat (although you will do this by handing the bag with your wrist strap on). Then, open the bag, remove the contents while wearing your wrist strap, and place the contents on the anti-static mat.
Do not take the board out of the bag without a wrist strap on, and walk it over to the anti-static mat. The act of walking with the board out of the bag can build up static charge. This will then cause a discharge when you place it onto the anti-static mat, possibly causing damage. Wear your properly grounded wrist strap every time you handle the board out of the bag, and only place it on a properly grounded anti-static mat grounded to the same potential as your wrist strap.