There is nothing more relaxing and calming for the body and mind than deep belly breathing, also called diaphragmatic breathing. Notice the movement of your belly as you inhale and exhale. With inhalation your belly will expand and fill with air and contract as you exhale. The contraction and expansion of the diaphragm communicates to the body that it is safe and it is ok to bring the cortisol levels down.
Here are two videos that feature two different breathing techniques. Try both of them out and see what resonates with you better.