You should be aware of both the breathing process and smell.
There is no fixed duration for this practice. It should be done until you experience a feeling of bliss, but don’t wait expectantly for 12 hours. You must use your discretion. A reasonable practice duration is from 5 to 10 minutes.
Manduki mudra should be done immediately after maha bheda mudra3 and before tadan kriya.
Position of eyes
The eyes should be focussed on the tip of the nose. If this is done correctly then the end of your nose will appear like a lingam. Fix your attention on this nose lingam.
Remember also that the eyes should be neither wide open nor completely closed. They must be half open, which is the same as saying that they should be half closed.
This kriya cannot be practised in darkness; it has to be practised in mild light, so that one can see the end of the nose. All the kriyas after manduki mudra can be done either in darkness or in mild light, depending on your preference.
Mooladhara chakra and smell
There is a definite relationship between the sense of smell and the mooladhara chakra. This is clearly stated in various yogic scriptures and one can also experience it for oneself. During the previous 7 kriyas mooladhara chakra has been gradually stimulated. This heightens one’s sense of smell. One becomes aware of very subtle smells.