Using your funnel fill the balloon with flour. Make sure to be holding the end of the balloon tightly to avoid a mess. Stop putting flour in the balloon when it reaches the neck. Don't fill the neck.
To make a stress ball, slip a funnel into the neck of a sturdy balloon. If you want to make a firm stress ball, pour flour, baking soda, or cornstarch into the balloon. If you want your stress ball to be softer, use lentils, split peas, or fine play sand. You can also mix a little rice into your filling if you like.
This simple tutorial for how to make a stress ball (or sensory ball) walks you through. I made a funnel out of paper and quickly filled five balloons with flour.