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You do not need to fill the flask. The less water you add, the faster you will be able to bring it to a boil. However, make sure that you do add enough water so that you don't boil it off too quickly. About 75 mL should do nicely. This will serve as a heat source for your water. Make sure you heat the water to the boiling point. This will force air to expand out of the top of the flask and also generate water vapor to fill the balloon. Remember that the flask is being heated. You should use gloves to avoid burning your hands as you secure the balloon over the opening of the flask. Make sure that the balloon is far enough down on the neck of the flask that it does not pop off easily. It may be easier and safer to put the balloon on the flask before heating the water. Securing the balloon over the top of the flask will create a seal and only allow the air to expand into the balloon. This expansion of air into the balloon will cause the balloon itself to expand. Do not let the balloon get so large that it pops. To prepare the ice bath just put water and ice into a container. This is a very easy and fast way to cool the contents of the flask. Use gloves to transfer the flask from the heat source to the ice bath. The rapid cooling of the gas inside of the flask and balloon will cause the volume of the gas to decrease. As the volume decreases, the volume of the balloon will also decrease, causing it to shrink. As the gas cools even more and contracts even more, the volume of the gas shrinks so much that the pressure outside the flask pushes the balloon completely inside the flask.
Add a small amount of water to an Erlenmeyer flask. Place the flask on a hot plate or burner. Put the open end of a balloon over the opening of the flask. Observe the expansion of the balloon. Move the flask to an ice bath. Observe the suction on the balloon.