Question: how to maintain focus and concentration on one thing ?

The path is the same as to Carnegie Hall — practice, practice, practice — with as much compassionate awareness, acceptance and forgiveness of wherever your monkey-mind goes as you’d give your own beloved child.  Non-judgmental focus or concentration is a goal, tho’ many meditators prefer to frame the Practice differently.  Learning to trust your own instincts and intuition is also a goal — whether or not a practicer terms it so, it is one of the desirable outcome most meditators intend.    Some pretend they don’t intend, but then, why practice?  Perhaps simply because it feels good-er than not practicing.  Dr Bob