Define the theme which envelopes the idea or message that the song conveys. For example, if the message were sorrowful the songwriter would likely choose a ballad type of feel to the song, typically constituted of a slow, sweeping rhythm and mostly minor chords.
Write lyrics that express your opinion or feeling toward a particular topic. This is entirely personal and the text can be worded as humble or as obscure as you like. There doesn't necessarily need to be a consistent rhyming pattern, as changes in flow add character to the sound of the vocals. The message of the song should be broken up into two or three verses, and an overview of the song's message should be used as a chorus that can be repeated.
Create a groove with the band's rhythm section. The rhythm section would usually consist of a drummer, a bass player and a percussionist. Each of these can be substituted, for example a drummer/percussionist with a DJ, and/or a bass player with a synthesizer player. The groove should be consistent with the song's theme. If the message is positive, motivational or exciting, the groove should make the audience feel like dancing, and this is most easily achieved with a straight-ahead rhythm in a 4/4 time signature.
Write a chord progression around chords that best convey the mood of the song. If the song is aggressive for example, power chords (which is the root note combined with the perfect 5th, or the musical interval between a note and the note seven semitones above it on the musical scale) will most likely be appropriate. The best approach when creating a melody is to use the same number of notes as syllables in the song's lyrics. The notes should all be kept diatonic (within the song's key) until the idea has been rehearsed a few times, at which point your ears and mind will imagine variations on the melody that might be more interesting. Song writing is after all an art form, and not a technical exercise. Match the lyrics to the melody, rehearse, and refine. Write a second chord progression to use as a bridge or to improvise a solo over. A simple approach is to use the same chord progression used previously, but in the relative minor of the key.
Combine the separate parts of the song into one complete arrangement. It is at this point that details such a drum fills and other transitional effects can most easily be added.