Let’s say we’re cultivating metta towards ourselves (this is the first stage of the practice). We start by cultivating lovingkindness towards ourselves because how we regard ourselves has a profound effect on how we regard others. If we dislike ourselves it makes it hard to have positive regard for others.
Using phrases is the classic method of doing the Metta Bhavana practice. I use this method more often than any other. There is no limit to the words or phrases you can use. The traditional phrase for the first stage would be “May I be well, may I be happy, may I be free from suffering.”
You have to say the phrase to yourself as if you mean it. You also have to remember to keep your focus on your emotions: you repeat the phrase, over and over, but watch what effect it has on how you are feeling. This is true for any word or phrase you use (and other phrases can be used).
Leave time between each repetition of the phrase so that you have time to absorb what effect it has. I often fit the phrase in with the rhythm of my breath. I say “May I be well” on one out-breath. Then for the next in-breath and the next out-breath and in-breath I tune into my heart and see what effect the phrase has had. Then on the next out-breath I say “May I be happy”. Then two out-breaths later I say “May I be free from suffering.”
When you’re thinking these words, you’re being active. When you’re listening for the effect they’re having then you’re being receptive. This practice needs you to be both active and receptive — actively working with your emotions, and receptively watching what effect your actions are having.
You don’t have to use that particular phrase. You can just repeat a word like “love,” or “kindness,” or “patience.” Or you can use a series of words.
Or you can remind yourself of what your own positive qualities are, and rejoice in your own merits.