Use the scissors to cut the top of the balloon off.
Wrap the top of the balloon tightly around the opening of the jar or can and seal it with the rubber band. Try to get the balloon as tight as possible and make sure that the rubber band is tight enough so that no air will get in.
Tape the stirring stick to the center of the stretched out balloon so that one end of the stick points away from the jar or can.
Place the jar or can next to a wall. Tape the index card to the wall so that the end of the stick is pointing to it.
Make a mark on the index card next to where the stick is pointing. You may wish to label the mark with the date and time.
Look at the card each day and see if the stick points above or below where it did the day before. If it points above the previous day's mark, the pressure is rising. If it points below the previous day's mark, the pressure is falling.