Building on this, once you have a rhythm, you can start to imagine what your lyrical structure and flow is like. Once you know how the words are going to fit in the rhythm, look for words that articulate the thoughts and feelings that went into the melody. Then find words that rhyme with the previous set of words, and string them together with the beat as a guideline for how many syllables you can fit.

Unfortunately, your first few goes are going to be either word salads that don't really make sense or pretty derivative of lyricists and poets you like. But a wiser fella than I once said "Sucking at something is the first step towards being sorta good at something."