To perform a stomach vacuum correctly, follow these steps: 1. Stand or sit upright, with a neutral spine. 2. Exhale all the air from your lungs by pursing your lips and blowing. 3. As you exhale, suck in your stomach as much as possible and try to touch your navel to your spine. 4. Hold the contracted position for up to 20 seconds (or as long as you can maintain proper form). 5. Release the stomach contraction and inhale. 6. Repeat the process for several repetitions. 7. Remember always to use proper form and build up your endurance gradually. Using some of the YouTube videos featured in this article can help you perform the exercise safely and effectively.