Just breathe calmly and deeply, and focus on your breathing.
When your mind wanders, refocus on your breathing.
Continue for 5 minutes, and after five minutes write down any thoughts are you haven't been able to clear from your mind.
Repeat every day if possible.
The results (for me) are that i can refocus on living in the present, rather than re-living events from the past (that can't be changed, but must be accepted) or worrying about possible future events (that may never happen). I find living in present to be less stressful.
Once this becomes a habit the cost of five minutes a day is low, and the benefit in greater relaxation and less stress is (for me) quite high.
hth