How can I harmonize a song effectively?

To harmonize a song effectively, you can start by listening carefully to the melody and identifying the key of the song. Then, experiment with different harmonies that complement the melody by singing or playing different notes that blend well together. Practice singing or playing the harmonies until they sound smooth and balanced with the melody. Experiment with different harmonization techniques such as parallel harmonies, counterpoint, or call and response to add depth and interest to the song.