139- Exercising the Senses
When you mentioned that the 12th sense in human beings is developing and evolving, it equates to humankind's evolution and the progression of their senses that we mentioned earlier. Plus, we also explained the 21-fold senses elsewhere.
Each sense's functions are different from one another, just as the olfactory sense is different from the somatosensory sense.
Now, how can this 12th sense (as you put it) or others be developed? This notion can be achieved by practice. If the practices and exercises are proper, the senses will strengthen.
Therefore, if you make your sense of clairvoyance work, that specific sense will start developing.