The felt sense. A thing which is usually peering outward. If nerves feel, why can't they feel IN?

Experience the skin, feel clothes or bare air against, the pressing of the Earth's gravity, being held. Palpably turn that outer eye in, look into the void within the body. Feeling perhaps for organs, physical sensation; feeling perhaps for emotion, sensation of the psyche; feeling for any subtle thing.

Keep away from labels. Once a label arises, reask, what is it? What is within? What am I feeling? Who is the one feeling? Find the edges from which you can grasp any known sensation, and peer deeper, past those into the unknown.

Look with a sense of ease, no need to achieve anything or do anying, just exploring.