Metta can be practiced in formal ways or creatively on the fly.Some suggest a balance between training one's mind and one's Heart (Metta and other Divine Abodes. (50% mind training/ 50% Heart practice. ...or Metta could be one's main focus/path)
The Four Immeasurables
as a Traditional Tibetan Buddhist Prayer
May all beings have happiness and the cause of happiness.
May they be free of suffering and the cause of suffering.
May they never be separated from the supreme happiness which is without suffering.
May they remain in the unlimited equanimity, free from both attachment to close ones and rejection of others.
Classic 5 layer method:
2 Benefactor, Dearly beloved/friend/close family member,
3 the Neutral person,
4 the difficult person,
5 All beings
Some suggest not going to the next level/person until some feeling of metta is felt.
Using phrases like: May they be well, happy and peaceful are used. We do this to the best of our ability (thus far).
Metta Sutta from the Sutta Nipata -
Pali Cannon translation/adaptation by Caitríona Reed
If you want to live in truth and integrity, if you hunger for joy and freedom—then live simply. Be impeccable in everything you do—in every word, thought and deed.
Be receptive, gentle, and unassuming.
Avoid irrelevant obligations and extraneous distractions.
Learn to be content with less, modest in what you consume.
Simplify your tastes, tread lightly. Be gracious and appropriate, self-sufficient.
Find ways to create balance in your relationships, especially with those people you love the most.
Respect the needs of others, avoid antagonizing them unnecessarily.
MAY ALL BEINGS BE HAPPY AND PEACEFUL!
MAY THEIR HEARTS BE FULL!
ALL LIVING BEINGS—strong ones and weak, large or small, seen or unseen, those nearby or far away, those already born and those yet to be born, beings you can only imagine, as well as those that are beyond imagination, wherever they are, whatever they are—
MAY THEY ALL . . . . MAY ALL BEINGS BE JOYOUS AND HAPPY!
Let no one deceive another. Let no one despise another.
Let no one out of resentment or anger wish harm to another.
Just as a mother would give her life to protect an only child, so cultivate that same boundless, open heart towards all of life.
Let your thoughts of unconditional love and kindheartedness permeate the cosmos, extending in all directions without obstruction, free from preconceptions or fear.
Whatever you happen to be doing, whether you are; seated, standing, walking, or lying down, so long as you draw breath, find ways to cultivate this quality of loving-kindness, this tender quality of mind, which of all qualities is said to most accurately reflect the truth of who you are.
Let go of any idea that you are separate from the web of living beings. Let your life be informed by the understanding that all things are interconnected. Find ways to relinquish what you have used as substitutes for love, and accept in their place the gift, and infinite pleasure, of boundless love.
Wake up to all that surrounds you!
Wake up to your life and be free!