Foremost is to understand Metta correctly and learn how to generate Metta in our minds. Then learn how to radiate Metta toward others.
Metta Meditation is not a type of prayer. During Metta meditation, we do not wish people various types of happiness. For example, we should not start wishing our subjects of meditation various good conditions, such as good health, wealth, romance, success, etc. Instead we should focus on generating a strong sense of genuine goodwill.
There is no "correct" sequence. During our meditation, we should focus on the flow of our goodwill (loving-kindness). Start with people who are easier for us to have Metta, then slowly work towards people who does not matter to us, and finally people whom we are averse to. Do not worry that there is a proper sequence to follow. Sometimes, our husband/wife may come into our mind first if we had a lovey-dovey moment the previous night. Sometimes they get thrown to the very last because we had a heated argument the night before.
The speed or pace of generating metta from one person to another should be consistent. Avoid lingering our Metta on certain people during the meditation. Metta should not lead to attachment but equanimity.
When generating Metta towards people, it need not be individuals but can be a group of people. For example, if we are averse to a certain race or skin color or sexual orientation, etc; Then we should attempt to generate Metta to them towards the end of our practice.
