Songwriters don't always get it right on the first try. They may tinker with a song for days, weeks or even years before it sounds as it should. The difference often is in pre-production, or the weeks leading up to a recording session. Artists and producers collaborate to rewrite or rearrange a song, or take a rough outline of a song and turn it into a masterpiece. Diagramming a song is the first step in this process.
Know the parts of a song. Though every song is different, most contain similar elements. These include introductions, verses, pre-choruses, choruses, bridges, solos and endings. Know what constitutes each, and how to identify each.
Listen to the song once for feel. Use headphones. This "big-picture" listen will give you a sense of the song's intent, mood and dynamics (the variation in volume between parts).
Assign the song a basic quality: Does it aim to be fragile, powerful, dark, energetic, lethargic, anthemic, catchy, weird, or angry? Or does it yearn to be something else entirely? Make sure you know the artist's intentions (a copy of the lyrics helps tremendously), and use them as your guidepost to tweak the song's arrangement and dynamic structure.
Listen again and diagram the song's arrangement. Songs are stories, and the best stories are told in a way that is concise, logical and full of character. Count out the length of the song sections, and note which seem too long, too short, too similar in volume, or in the wrong order. Common mistakes include overlong introductions, incongruous middle sections and dynamic monotony (playing at the same volume and energy level throughout.)
Identify the song's hooks, or its catchiest and most memorable parts.
Suggest sections to trim, lengthen, cut, change and re-arrange. Use the song's basic quality to inform your decisions. For example, give an energetic punk song a short, punchy introduction; emphasize the choral melody of an anthemic song, and isolate the vocals and main instrumentation of a solemn or fragile song. Preserve or emphasize the hooks--they're the parts that make the listener want to hear more.
Consult the masters. If something doesn't sound right, listen to the bands that defined or perfected the artist's genre. Know their sense of dynamics and arrangement, and incorporate those elements into the song. Many a rock band has made a better song after a producer told them, "We could do a very AC/DC-sounding thing right here."
Trust your instincts above ironclad rules. The basic rule of music is that if it sounds good, it is good. Know that great songs sometimes follow unconventional arrangements, but keep in mind that they are the exception to the rule.