Notice what is roughly predominate in awareness and label it, about once per second, as either seeing, hearing, feeling, or thinking. Or don't know.
The structure there is based on the 6 basic sense "doors", which are the 5 traditional senses plus thinking. Within the 5 senses smelling and tasting have been dropped for simplicity and because those don't tend to come up that often in formal meditation.
Notice that "seeing" is the note even if your eyes are closed, if your attention is on the visual field. Same for "hearing" even if what you are hearing is imagined.