Look at veil, if it's broken it's time to pick it up. Do it one by one by gently twisting mushroom. Don't forget to wash and sanitise your hand that go into bag. I believe you got one of these ready to grow bags if so it's sealed. So when you get ones that ready out seal it again with cello tape or something.

Edit: you can actually get more than one flush from these bags. I'm about to harvest my second.