This exercise involves concentrating all your attention on the physical feelings of breathing in and breathing out, without deliberately changing the rhythm of your breathing.
Focus attention on feeling the cold air entering your nostrils and the warm air when you exhale. At first you may become self conscious about your breathing and find it becomes irregular, but this does not, as a rule, continue. If it were to do so, and you find yourself becoming breathless, then this exercise is not for you at present.
Most people find that on doing this exercise the pattern of their breathing changes, the breath becoming deeper and slower, and they begin to feel drowsy. In itself, it is a very good relaxation exercise, but if you care to use it for more explicit prayer, then let the inbreathing express all that you long for in life, however impossible it may seem in practice, and let the out-breath express your surrender of everything to God, all of your life with its worries, sins, guilt and regrets.
It is important to do this without self-judgement, whether of approval or disapproval. Keep your attention fixed on your desire to hand over all these worries about self, and do not clutch at them as if they were a treasured possession.
(adapted from God of Surprises by Gerry W Hughes SJ)