To perform this meditation it helps to be in a suitable environment and maintain a correct posture.
Find somewhere crowded, noisy, hot, stressful and generally frustrating. A packed commuter train or bus is ideal.
Check whether you're sitting comfortably. If so, you're doing it wrong. Get up off your ass, hang on the strap and let someone who needs it have your seat.
The correct frame of mind
Concentrate on your own frustration, stress, boredom, discomfort and low-level hostility to the passengers around you who are swaying and bumping into you and crowding you in. Visualize these negative feelings as a cloud of acrid black smoke swirling around your heart.
Next, imagine all your fellow passengers are experiencing the same hassles.
On an in-breath, breathe in all their irritations as black smoke which mixes with and annihilates your own swirling black smoke in a burst of white light.
The light purifies and calms your body and mind. Imagine the irritation of yourself and the irritation of others as antimatter meeting matter in a burst of mutually assured destruction, producing vast amounts of pure positive energy.
As you breathe out, imagine this pure white energy radiating out into the hearts of all those around you, remaining in the form of a globe of white light, an abiding centre of calmness and clarity in their hearts.
Now repeat the procedure, but this time the energy release is going to be HUGE, like a karmic supernova.
Imagine all the problems and sufferings of your fellow passengers: sickness, pain, sick relatives, financial worries, work problems, loneliness, insecurity, fear, relationship breakdowns, legal hassles, mental illness, addictions and so on. Then reflect that you yourself have the negative karma to experience all this, and much worse, for life after life. This negative karma swirls around your heart like a thick cloud of hot, acrid, black smoke.
On the in-breath, breathe in all these present and future sufferings of your fellow passengers in the form of black smoke, which mixes with, and annihilates, the negative karma at your heart in an ENORMOUS release of energy in the form pure white light. This reaction utterly destroys the sufferings of others and your own bad karma like matter meeting antimatter.
On the out breath, imagine that this colossal burst of purifying energy radiates in all directions through everyone around you, cleansing all their negative karma, and their potential for future sufferings, and sweeping it all away, utterly destroying it never to be seen again.
Finally, dedicate any merit you may have created from this meditation to the enlightenment of all sentient beings.