When mind is agitated because of anger or fear then breathing becomes shorter, shallower, fast and irregular. When mind is relaxed and calm then one breathes slowly. Mind and breathing are interrelated. If one of them is controlled then it will regulate the other automatically. For calming mind, a spiritual aspirant can practice bhastrika, kapalabhati, jalandhara bandha, uddiyana bandha, mula bandha, anuloma viloma, suryebedhe, chandrabedhe pranayamas. Pranayam has four steps poorak, antarkumbhak, rechak and bahir kumbhak which should be performed in 1:4:2:2 ratio.
(Excerpt from Japa Meditation - The Easiest Way to Calm the Mind by Chandrika Shubham)