I presume you mean how do you stop your mind from wandering? Assuming this is the question my answer is as follows.
Any attempt to force the mind from wandering will not only be fruitless but may also increase your mental wanderings.
Proceed gently and patiently, allow the mind to wander, force and effort 'to control' will only create emotional and psychological resistance to achieving what you wish to achieve.
Concentrate for as long as you can concentrate without any effort to control, over time with regular practice (small & often like a good diet) your mind will wander less and you'll be able to focus for longer. Your mind doesn't like to be told what to do (who does) it needs to be coaxed gently. Invest the time gently and patiently and your concentration will improve.
Don't concentrate too hard, this in itself will create resistance to concentrating, allow your concentration to be relaxed and fleeting, allow the commentary in your head to go where it may but maintain a small relaxed part of your mind at the task in hand.
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