I've been doing my own version of mindfulness for a couple of years now. Deep breathing definitely helps me to calm down. And trying to live in the moment and concentrate on the task in hand is useful. I no longer wonder if I've forgotten to turn off the gas, whatever, because I concentrate when I do it. I even say it out loud, "I've turned off the gas".
When relaxing, I start with my feet. This is where I am, this is where I am rooted in the here and now. Not yesterday, not tomorrow, just here and now. Then gradually relax all my muscles in turn, do steady breathing, and try to think of only one thing. A flower, a vase whatever. Just a few minutes, a few times a week. It then becomes a habit.
I realise that my version of mindfulness is probably rubbish.
But it helps me to deal with stress. It really does. I'm having less attacks of depression these days.