When we feel big emotions, "Take a deep breath" is common advice that often doesn't work.
It's vague and ineffective.

Googling "deep breathing," on the other hand, results in an overwhelming array of options, some quite complicated and hard to remember.
Counting and timing breaths isn't easy when we're feeling intense emotions:

"Breathe in for four seconds, hold it in for two seconds, then breathe out for eight seconds. Oh, and remember to inhale through your mouth and exhale through your nose."

It's not exactly panic-attack-friendly

Let's simplify matters:

Instead of counting or timing breaths, just breath as deeply and slowly as possible.

What you do with your thoughts is also important.
Try focusing on how each breath sounds and feels in your body.
Refocus on your breath when distressing thoughts return to your mind.

Keeping it simple, but not too simple, can turn this potentially futile exercise into a highly effective one.