Using the funnel, fill the balloon about halfway with baking soda. If you don't have a funnel on hand, you can easily make one from a piece of paper: fold it into a cone and tape it to prevent it from coming apart.
Again using the funnel, pour all of the vinegar into the bottle.
Without letting any baking soda enter the bottle, stretch the mouth of the balloon over the mouth of the bottle.
Lift the balloon so that the baking soda falls into the bottle and mixes with the vinegar.
Observe the reaction at work, inflating the fizz-flator. Why do you think this is happening?