Start by slowly stretching the balloons so that they'll fit on the soda opening. Don't put your mouth on the balloon beforehand as this will inhibit the experiment.

Once that is done, put the balloon over the funnel opening and pour in a pack of the Pop Rocks. (tap gently to get it to go down into the balloon or use a skewer to gently push them down)

Take the tops off the soda and put the balloon over the top. Make certain that the tip of the balloon covers the opening completely. (and be careful to not let any candy drop in quite yet)

Once secured on the soda bottle, tip up the balloon and gently shake the Pop Rocks down into the soda. The gas will start to form right away and the soda will begin to fix. You'll hear the candy start to crack and the balloons should start to fill up with air!