Exhale completely - you should begin to sink in the water. If you don't sink, try to exhale even more. If this still doesn't work, you need more weight - add a unit of weight and repeat the exercise from Step One.
It's important not to kick your fins while exhaling as this can push you up and make it seem like you're under-weighted when this is not the case. Try to keep your body quite still while performing this exercise.