The breathing technique I use is called 7:11. Breathing deeply (i.e. ensuring from diaphragm - so belly moves too, not shallow breathing from top of lungs only), breathe in to the count of 7, and breathe out to the count of 11. The aim is to make the out breath longer, as this engages the bodies natural relaxation response.
If these numbers are too long, can adjust - for example count to 3 in and 5 out, as long as out breath is longer
Keep doing it for a few minutes at least
If dealing with panics, it helps to practice this breathing at other times, so when you feel a panic rising it will feel familiar to do and easier to remember at the time (try and practice for a few mins to 10 mins, whatever feels right for you)
If you are quite a visual person, could find a rectangle in your surroundings (window frame etc, be creative), and scan your eyes around the shape at a steady pace, breathing in on short side, breathing out on long side. That can help to add distraction, if you can distract your mind that can help keep the panic away too
I suffered panic attacks last Christmas when I was ill, and this helped so much. I found it sometimes helped to get out and do in fresh air too (stepping out in the garden) the more you do it, the more you feel in control of your body and reactions, which also helps control panic attacks too
xx