Emo music is typically based off punk styles of music. Rather than using the more decadent or political lyrics typical of punk, emo bands focus on emotional, expressive lyrics. The simplicity of most emo bands does not preempt the need for good songwriting. Simple use of dynamics can create a solid, meaningful vehicle for the lyrics of the song, emphasizing the emotion behind them. A few simple songwriting tricks can make a huge difference in creating a good emo song.
Writing a Good Emo Song
Decide on an overall feel for the song. This is the basic emotion the song should invoke in the audience. This is usually a good time to write the lyrics as well.
Come up with a basic chord progression for the song. The use of I, IV and V chords tends to be common in emo. The progression is usually written around four repeated bars and each chord is at least one bar long. A common structure is I-I-IV-V or I-IV-V-I.
Create the verse instrumentals. The bass player can likely play root eighth notes for this. The standard rock drum beat will usually work well (1, 3 bass drum and 2, 4 snare). The guitar will usually play straight eighth note power chords or octave notes.
Create a louder, more aggressive version of the verse for the chorus. The bass player might still play the same part as the verse. The drummer will play more aggressively, such as going to double time, adding off beat strikes or adding fills. The guitar should switch to a louder dynamic of the verse part, such as removing the palm mute that is common in verses.
Write any melodies or solos. This is the time to add the vocal melody. If a guitar melody is used at all, it should be a simple, straightforward part. Typically instrumental melodies will be using notes off the minor pentatonic scale.
Structure the song parts as written. An example of this would be intro, verse, chorus, verse, chorus, solo or breakdown, verse, verse, chorus. This is a simple, recognizable song structure that works well for a less instrumental genre, like emo.
Play the song as a whole. The song should flow together. If it does not, then you should tinker with the song structure until it fits better.