There is no such thing as perfection. Not entirely. Not, at least, in any human endeavor. And translators have been hated for centuries, because translation is not perfect either. Not in full. There is always room to argue. All translations are, at best, an approach. And that is especially true when translating poetry and lyrics.
Know Your Author
We have already established that, before translation, comes research. Regardless of the field in which you are working. In Poetry, you need to know the author in depth. Do not limit yourself to the specific piece of writing you will translate. Read as much as possible, considering your schedule and deadlines. It is vital that you familiarize yourself with the style, tone, rhythm, voice, use of literary resources, recurrent topics, and even biographical notes.
Rhythm, Rhyme or Reason
If there were an arbitrary order, we would proceed first with the structure of the poem or lyrics. First analyze the types of verses, their metrics, their rhythm, and the kind of rhyme present. Make sure to comprehend, first of all, the actual wording, and literary resources. Pay attention to the poet’s expression, tone, intention, and artistic use of language. If there are words you do not know, research and record their meaning.
Both in poetry and song lyrics, rhythm and rhyme are the royal couple. Achieving a completely faithful translation, including rhythm and the exact type of rhyme is, at best, quite a feat. Word extension is different between English and Spanish, musicality differs as well, and rhyme could eventually turn into a bad dream. Nevertheless, we strive on. It would be ideal to maintain the original rhyme scheme. Due to the differences between both languages mentioned above, that is hardly the case. If it is achieved, the odds are you will be losing meaning or natural word flow. Something is always lost in translation, whether colleagues care to admit it or not. Especially in a field such as poetry and lyrics, where language is impacted by subjectivity. In any case, a solid background in literature and poetry in your native language will be an invaluable asset for the task.
Staying True and Flying on Its Own
While translating poetry, you will be walking on tightrope. Constantly. There is no safety net below. And you will grow to love that. Sometimes, finding the closest option for rhyme and rhythm is impossible. Then, just accept it and move on. You can reinterpret artistically the most complicated verses, and offer an adaptation that is much more than decent in Spanish. Sometimes that is all you can do. Choose your battles. There are a few other routes you can take, such as assonance, near-rhymes or transcreation. Your aim should be to achieve verses that resonate in Spanish, while remaining true to the spirit of the original piece. Then, let your work fly on its own.
The Ability to Transfer Emotions
The hardest part of translating poetry and lyrics is being able to transfer emotions across both languages. The ideal translation should evoke the same emotions and feelings as the original poem. This is when being a poet herself/himself would come really handy for the translator. Only this trait gives you the right kind and amount of empathy with the author, and the source material. That is, at least, to have a fairer shot at a more loyal, and heartfelt translation.
When all else seems to fail, surrender to the source text with all your technical resources and craft tools. Embark in a deep reading of the poem, and allow its power to overwhelm your own words. Let it rest for a bit. Then come back, and let the English poem flow through you. It will be recreated thanks to your skills and, partly, through a magic of its own.