Most breathing advice sounds like it was written for someone sitting quietly in a garden. You are probably not in a garden.
This works anyway.
Why it works
The key is making your exhale longer than your inhale. When you do that, your vagus nerve signals your heart to slow down. It is one of the few ways to manually override your stress response.
The technique
Breathe in for 4 seconds, out for 6. That is it. No special position, no closing your eyes if that feels weird. Just the ratio.
Do it for about 90 seconds: roughly 9 breaths.
If 4-6 feels forced, start with 3-5. If you feel lightheaded, go back to normal breathing and try a slower count next time.
What to expect
You will probably not feel dramatically different. What most people notice is that their chest loosens slightly, or their thoughts slow down by one gear. That is the shift.
One thing that helps afterward: notice one concrete thing around you. A sound, a texture, the temperature of the air. It keeps you from sliding straight back into your head.