S Breath
Take a deep breath in, then exhale gently in a SSSSS S-breathe (hissing like a snake). Try to keep your exhalation as a controlled, steady and constant stream for as long as you are able.
When you start to feel the urge to breathe, try and exhale just a little bit more before you have to breathe in.
Some people can manage just a few seconds, others can manage around a minute.
Do what is right for you to begin with. Try timing yourself. Eg this week you managed 10 seconds, next week try for 11 seconds etc.