Those familiar with the Mindfulness-Based Stress Reduction body scan or the Goenka method of scanning body sensations from head to toe will see some similarities to this practice. Sense-base acuity practice is a variant of contemplation of the sense-bases (ayatana) described in the Satipatthana Sutta as a dhammanuppassana practice, one of the fourth foundation of mindfulness.
Begin by focusing on seeing, either with eyes open or closed. You will be aware of sights before you if your eyes are open or, if your eyes are closed, variations of light and shadow. Focus your energy on exercising the organ of sight or the "eye sensitivity" by trying to see whatever is before you without identifying or associating to what appears, all the time noting, "seeing, seeing, ...." If no sight is particularly present, be aware of the eye sensitivity itself. After a few minutes, switch to hearing.
Focusing your energy on exercising the "ear sensitivity" by detecting sounds that occur, all the time noting "hearing, hearing,...." If no particular sound appears, be aware of the ear sensitivity. Again, after a few minutes, switch to smelling.
The dominant senses during eyes closed sitting meditation are hearing and touching and, if your eyes are open, also seeing. Smelling and tasting are not usually as evident and are best practiced during eating meditation. However, focusing energy on smelling while doing sitting meditation may reveal subtle smells or the absence of smell, in which case, awareness of the sensitivity may be effective. Focusing on tasting in sitting meditation is facilitated by briefly swallowing and moving the tongue around the mouth. You may detect various tastes. You can then move on to the next sense base.
Focusing on touching sensation opens up a range of bodily sensation. Begin by noting the whole body and its contact with the surfaces it touches, go then to dominant sensations that are present and, if evident, to the rising and falling of the abdomen or to the sensations of the breath at the nostril. Again, after a few minutes, shift to noting thinking.
Putting energy into noting thinking may effectively dispel whatever thinking is occurring. Try to detect nuances. Is the thinking past or future focused? Is it tinged with emotions? How does it relate to physical sensations that may be occurring?
The final step is simply generalizing the energy you have developed through the exercise and opening up to whatever sensations occur. You can also return to a breath focused practice. You can do this as long as you do not drift off into excessive wandering or thinking that goes unnoticed for lengthening periods of time. When this occurs, you should begin cycling through the six senses again. In my experience doing this practice only a few times during a period of sitting meditation generates enough energy to keep me focused throughout the remainder of the sitting and the effects of the practice sometimes even carry over to subsequent meditation sittings.