Anapanasati practice: what’s the correct way to focus on breath?
There is no need to focus on the breath. 

The practise of the Buddha is to keep the mind without craving, without attachment, without past, without future. Silent still mind. 

When the mind is quiet, it will connect with the breath, itself, automatically. 

Experiencing the breath is merely the "sign" the mind is without craving. 


  And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, making it his object to let go,
  attains concentration, attains singleness of mind.
  
  Indriya-vibhanga Sutta