You just focus on the breathing sensation, the flow of air, without breathing heavily, following the inspiration, the slight pause and then the next expiration. Again and again.
Your mind will obviously create a lots of "shower thoughts", the goal is to come back "gently" to the focus on the breathing. By gently, I mean you should not feel angry for having your mind wandering, this is natural and expected. The idea is not to block or shut down your mind on any idea arising, but rather when it occurs trying to put them aside, and go back again to the breathing.
It sounds simple and easy, but in practice it is hard to maintain your attention for an extended period of time.
I think in Buddhist tradition it is a part of Samatha [0]
I believe there are some variations like focusing on a candle, or a stone, the advantage of the breathing is that is available everywhere at anytime.
[0] https://en.wikipedia.org/wiki/Samatha