The thing about Metta is we have to make a quick observation of our existing emotions before we do it, and this requires a little bit of housekeeping before doing it. So quickly to imagine the issues that might come up, examine the things that make up your practice. First examine your environment, sometimes because of how you feel is affected by how the world is around you, check your surroundings. Is it cluttered? Clean it up and see how it affects your mood. Is it dark or dim? Light up the room and see how it affects your mood. Is it noisy? Go to a quieter place or play white noise see how it affects your mood. Is your job and family driving you crazy? Take a retreat, go to the park and see how it affects your mood. Heck, just one visit to an animal shelter to play with puppies changed my ability to muster up metta one day when I was upset. Sometimes when we're upset we have to go to a novel place or take new steps in your life to build up a change of pace.
Next patiently examine your body and physical sensations. A strong but comfortable posture might change how well you are able to do metta. If you're used to doing metta and not feeling enough building up, change your position and posture. If you usually sit, stand up or lie down instead of sitting and examine how this affects your emotions. Practice relaxation of the body and let your body be as relaxed as you can, almost like you are feeling the warmth of your head melt butter and as it rolls down relax every muscle. Sometimes a body scan can reveal tightness and discomfort. It's pretty hard to wish metta if your knee's cramping, your leg's asleep and your nose itches. Wait until the body is relaxed as you can get it, and then begin the practice.
Then take into account your thoughts. Your inner narrative might go, "May, I be happy" and then immediately go, "MAY? That's so weird, I don't talk like that this feels fake, oh NO... it's FAKE!" and you'll feel discouraged and the concentration breaks. Don't get stuck on things like words, change it to your own words. It can vary anywhere from the traditional "In safety and in happiness, may I/they be at ease", to a simple positive affirmation "They/I will be happy!", just make it your own, make it sincere, and patiently let the self-defeating thoughts go. Once you use your own words and feel the way it is meant to feel, it'll come naturally. Your awareness of thoughts gives you a choice here to be negative or to strive on. Reflect on the words you're choosing. If you're doing the "May they be well, happy, and free from suffering" be specific. Look into your mind and think of what it would mean for the person to be well. What would it mean for the person to be happy? What would it mean for the person to be free from suffering. Make sure you figure this out BEFORE practice of metta.
Then if all of these things don't give you a definite change then talk to someone about what's hurting you, what is making you mad, what's making you upset. In so many cases it's because you're holding in resentment and pain that when you try and do Metta you're left at a loss for what to do. Sure you cherish your friend X, but lately you feel Y and it's interrupting your ability to get the Warm And Fuzzy Feelings (WAFF) required to get the metta going. Talk to someone, a trusted friend and get comfort. It'll give you a wave of positivity and a chance to reframe your pain so you can instead practice without hangups.
Now that was the housekeeping, you're ready for metta? Let's go.
You can try using words, deeds, and thoughts to help you cultivate Metta. Just like before you're aware of your body and patiently be kind to it. Make positive affirmations to yourself. "May I be well, may I be happy, may I be free from suffering." Now a lot of people are taught to not like themselves, but if self defeating thoughts come up, roll your mind into a flood of positive self-affirmations. You're doing this because you value the happiness of others, and your selfless good will makes you a worthy individual. The will to free yourself from suffering in samsara as a disciple of the Buddha makes you a worthy person. Cherish that truth within yourself.
Then think of a good companion. A friend. Think of all those times you just felt great about being with your friend. Think of a specific memory where you were happy and enjoying the best and most wholesome things about friendship. Then the words seed in, "May they be well, may they be happy, may they be free from suffering." (classic example, remember use your own words seeded with the meaning you gave it and examine the exercise). Remember we believe in kamma; every word, deed, and thought releases conditions that would make this happiness, wellness, and freedom impossible. So imagine you're actually seeding in the conditions yourself into the universe by your practice and will this happiness for your friend. It becomes less hard to imagine it as concentrated good will meant to sow a seed of change. Then let this feeling emanate in all four directions, above and below, and all around. Once you feel like this has permeated outwards, expand it further. Think of someone who is neutral and do the same thing. Then think of a person who you've been feeling difficulties with. Do the very same thing and then expand beyond your social circle. Let the metta go beyond everything you even know. Then "May all beings be well, may they be happy, may they be free from suffering."
And of course the very sutta mentions how it should be done in detail!