Start by pouring your flour or corn starch into the empty 2 litre plastic bottle (make sure the bottle is dry before you pour in the flour). Place the bottle upright on a flat surface.
Next, fit the mouth of the balloon over the rim of the plastic bottle. The balloon does not need to be filled with air – simply use an empty balloon for this purpose.
Now, squeeze the bottle gently – this should cause some of the air in the bottle to partially inflate the balloon. Hold it this way to keep the balloon inflated.
Keeping the bottle pressed, turn it upside down, to pour all the flour or corn starch into the partially inflated balloon. Shake the bottle to make any loose corn starch fall into the balloon.
Now, relax your grip on the bottle and stop squeezing it. Shake gently to dislodge any excess flour in the balloon, but do not take the balloon off the bottle yet – allow the flour to settle and any excess air trapped inside the balloon to escape.
Once the flour inside the balloon has had some time to settle, squeeze the end of the balloon that is attached to the bottle rim, and turn the bottle right side up. Still squeezing the end of the balloon, pull it off the bottle, and tie a knot as close to the filling as possible. Your homemade stress ball is now ready.