The instruction is found in Kevin Griffin's One Breath at a Time:
Very slowly move through these phrases in your mind, contemplating their meaning and letting their meaning penetrate beyond the idea to the actual feeling itself.
"May I be appreciative and grateful."
"May I be aware of beauty and joy [in others]."
"May I be open to beauty and joy."
"May I respond to beauty and joy with appreciation and gratitude."
Think of those who are dear to you and offer them the same wishes. Say their names to yourself as you repeat the phrases, "May ___ be appreciative and grateful." Envision them experiencing mudita.
"May___ be aware of beauty and joy."
"May___ be open to beauty and joy."
"May___ respond to beauty and joy with appreciation and gratitude."
After wishing those who are dear to you mudita, move out to people more neutral: neighbors, colleagues, people you see in your daily routine. Instead of using names, you can visualize them and say, "May you be aware of beauty and joy." You can see many faces as you repeat the phrases.
Finally, wish mudita for those who are difficult, people you resent or fear, or someone who has harmed you.
Then radiate mudita outward to all beings nearby, and gradually out to the whole planet, and finally the entire universe.
Here is a link to an article on mudita you might like to read. And here's another. Both articles bring joy just by reading them.