Sit somewhere quiet where you can be alone and undisturbed. This exercise can be done either with your eyes closed or else open and resting on a simple object such as a flower or candle flame.
Focus exclusively on your breathing. Slow your breath down and make the flow of breath in and out through your nostrils as calm and controlled as possible. Imagine that someone has placed a tiny thread right in front of your nose: focus on that thread and ensure that it is absolutely still and unwavering, so smooth and gentle is your breath.
When your breath is calm and controlled, please imagine that what you are breathing in from the atmosphere around you is not air, but peace: solid, tangible, abiding peace. This peace is all around you and you are breathing it directly into the very depths of your being, where it starts to accumulate. You can imagine there is a vessel inside you, which is gradually being filled with peace.
When you breathe out, exhale the opposite of peace: restlessness, tension and stress. Just let go of these things: they are not yours and you have no right to hold on to them, so let them go, release them, allow them to flow out of you.
Imagine that you are breathing in this peace not only through your nostrils, but also through your eyes, ears, mouth, through the very pores of your skin.
Imagine you are a dry sponge which is soaking up and absorbing peace eagerly and thirstily.
After a while you will have breathed out all the restlessness and stress and your inner vessel will be full to the brim with peace: at this time you are now breathing in and breathing out peace.