Find a comfortable chair and sit straight. Close your eyes, preferably. If you rather keep your eye lids open, pick a spot in the room that you are in to focus.
Now let's address your breath and the associated rhythm. I would recommend thinking not so much about your breath, but rather think about a place on your body which the breath affects, e.g., the in & out movements of your anatomy affected by your respirations (expanding your ribs, filling your lungs. Just feel it. You could also say, "in" and "out" with each cycle. Invariably, your mind is going to start to wander, what am I going to make for dinner tonite, don't forget to pick up the laundry on the way home, don't forget to tell my partner this or that. This is a normal process. We used to refer to this as day-dreaming. When this occurs, don't judge yourself, just bring yourself back to the mechanics of your breathing...in & out. Three minutes, 5 minutes...it's all good. There are some wonderful 10-minute meditations on You Tube. A daily practice of that is like a tonic for your soul. Feelings related to stress will just melt away.