Adopt a comfortable position, but don't slouch - you need to be alert. Focus on something of which you are consciously and actively aware; for example, your breathing. Focus on it, and pay direct attention to it. Don't try to analyse it or explain it (Biology teachers, take particular note) but simply experience it as a part of your life. Even if you're not a Biology teacher, you'll know that this breathing - this simple 'in' and 'out' - is a kind-of-important part of your life. It is worth knowing about, being familiar with, and appreciating.
Focussing on this one experience usually involves reducing attention in other areas, and ignoring some of the many stimuli which the world is throwing at you. Just concentrate on the 'in' and 'out. You may find your mind starting to wander. That's natural and OK, but just gently ease it back to your breathing. 'In'. 'Out'.
While doing this, there is one thing (well two things really but we'll treat them as one) that you need to NOT do. These are to revisit and imagine what has gone before ('I should have told the Head I was too busy...') or to anticipate and rehearse what has yet to take place or will never take place ('Wham! Take that. Bloody parents. Wham! And that...').
The whole point of mindfulness for teachers - in the context of time management anyway - is to avoid wasting time and effort on pointless fantasy, and to focus clearly on what is happening now. The intention is that by removing negative, stressful or value-less thoughts, you can improve both your quality of life and your mental health.
If you can effectively concentrate on the here-and-now - not what might happen or what has already happened - we can develop a more acute awareness of how the here-and-now is affecting us; affecting us both physically and mentally. Elsewhere on this site you may have read about the 'rattle bag' that is a teacher's mind: a jumble of students' names, targets, ethics, issues and things to do, of last week's meeting and next week's exams. There is just so much going on. Too much in fact, to be helpful or healthy.
The purpose of this exercise, of this learning, should be simple and clear. You are trying to develop the skill of living in the now; of the present moment. Memories and plans should be filed away. All that matters now is what is happening now. And what is happening now is this: 'in' and 'out'.
Now if, like most teachers, you possess a streak of sensible cynicism, you may be considering that - as a route to mental health and greater happiness - this in-out business is pretty dull... and guess what? You're absolutely right. We are conditioned, in modern society, to be stimulated constantly, to be peppered with distractions and diversions, to plan and reminisce at the same time as we act, and to do several things at once while thinking about several things more. It is hardly surprising that - at first - focussing on one single aspect of reality may seem a trifle humdrum.
However, the fact is that you cannot - physically can not - have it both ways; you cannot have both calm and excitement at the same time. You may find tranquillity boring until you become used to it. The good news is that when they are accustomed to inner peace and calm as alternative ways-of-feeling, very few teachers have any regrets or nostalgia for a life of constant frenzied over-stimulation.