Simply pick your nose in reverse - apply sparingly into the nostrils:
Simply squeeze a small blob onto your little finger, and wipe it inside your nostril
(a bit like picking your nose in reverse).
Or introduce the nozzle just inside each nostril and squeeze a small amount out. Then gently squeeze your nostrils together (as when you try to stop a sneeze), and wriggle the Nisita around inside your nose for 4 seconds.
You get some funny looks when you do it! Try doing it in the bathroom /restroom/WC/ behind a book/etc etc
Don't forget to do the other nostril as well!
There are no hard and fast rules about frequency, but we use it every two hours on a plane, or just twice when skiing /mountaineering/walking and every two or three hours in the office.