No need to overthink any special technique. Just breathe slow, and try not to breathe too shallow. Do it as an exercise for a few minutes. Breathe with the stomach (not chest), like a baby would.
One book I particularly recommend is “the healing power of the breath” , the exercise is very simple, basically just establishing a rhythm that works for your body type, and then you are good to go.
Thats not to say there’s not value into the pranayamas (breathing exercises) of yoga, but for pure efficacy just establishing a good rhythm is actually easy to learn and use.