The plastic bag is to provide extra humidity (like a makeshift greenhouse) to help speed the rooting process. I tried that method a couple of times but it's very humid where I live and everything turned to mush quickly so I discontinued using plastic. You can leave the little pots inside the bags until you see new little plantlets arising from the mother leaf. Just be very careful when watering, whether the pots are covered with plastic or out in the open. If using the plastic bag method be sure the plastic does not touch the leaf. I used to take a short glass and turn it upside down in the bag to hold the plastic above the top of the containers ... wooden skewers work well too and are inexpensive. I used to buy packs of them at the dollar store and I think there were a dozen or more in per pack.
