If you are looking to cover the song or just appreciate the technique:

Roca used a smooth baritone with a lot of "vibrato." To mimic this, don't rush the phrases; let the ends of the words shake slightly with emotion.

The song is a dramatic, emotive ballad. It tells the story of someone watching an ex-lover move on, questioning who could possibly love them or understand them as deeply as they did. It’s heavy on sentiment and "feeling." 2. How to "Do It" (Performance Tips)

The verses are usually soft and conversational. The power comes in the chorus (the "Quién, como yo..." part), where you should increase your volume and intensity to show heartbreak.