The instruction is to feel the touch or strike of the breath/wind/air against the flesh of the inner nostril, where it is most sensitive. If this touch in the nostril is not clear to your mind, then place your finger gently against your upper lip below your nostril and feel the touch of the wind/breath against your finger. Keep your finger there until you can feel the breath against the nostril & finger so the in-breaths & out-breaths are discerned/recognized clearly & unconfused. It is important that the mind here is unconfused about when the breath is coming in and when the breath is going out.
This practise narrows the awareness & thinking of the mind. By doing so, it also makes the breathing very fine & calm , which makes the mind calm. This is the purpose of the practise.
However, for the beginner, this deliberate narrowing of mental focus & refining of the breath can eventually make the mind sleepy.
A beginner can experiment with & practise this method of observing the touch of the in & out breaths at the nostrils. It can bring good results of calmness.
However, if the mind becomes sleepy or 'sinking', the beginner should open their awareness and return to observing the breath within the body, particularly the flow of the breathing & its movement in the throat, chest &/or abdomen.
Again, if the flow & movement is lost here, place a hand on the abdomen & another hand on the chest to clearly feel & know in-breathes & out-breathes in an unconfused manner.