First of all I have not written any songs with lyrics for at least 5 years, and also I only ever made a single album with a band, which was actually the "Song Writing and Recording" course students I was learning with. In that short 8 (or so) week course, I learnt a lot of things about writing music but even more about myself. Writing and playing music is not just about the technical side of things or even just the outward expression of ideas through music and lyrics. It is also about the self-exploration, so anything anyone ever tells you about "how to do" something that is all about expressing yourself outwardly should never be treated as gospel. Playing from the heart means playing from YOUR heart, not mine. I didn't know whether to reply to this thread because I really don't have a huge amount of experience in song writing.. but as only got one reply so far, I thought I would give my opinion too..

It really depends on the purpose of the song to me. If I am writing something that is more to do with the way the music sounds, then I will always start with the chords then build the music up around the chords. Then I usually fiddle around with different timings using that same chord progression and find something that not just sounds good but also invokes some sort of image in my mind. It is at this point that I have a general idea of what the lyrics are going to be about. Then, with the chord progression in it's "perfect" timing in your MP3 player, you can put down the guitar and start humming melodies while meditating / concentrating on the image, feeling or idea that you had earlier. At some point your mind will just fill in the blanks, and writing the actual lyrics becomes much less of a chore because you know the timing and the melody you are trying to achieve. The lyrics will bend to fit themselves into your existing beat.
If it's all about the lyrics, then write out your "poem" and apply it to a melody and a timing. Once you are singing the song with ease, the next step is to create the music side of things. However, instead of immediately picking up the guitar and playing.. you should find out the timing and tempo and all that so that strumming the guitar does not overtly affect your original idea. I find that as soon as I am strumming the guitar, I lose sight of my original timing, melody and tempo. Best to get out a metronome whether analogue or digital, or better yet use some form of drum machine to create the exact feel you want. Only then should you pick up your guitar and start working out the chord progression.

And then there is the third option.. just make it all up as you go.. not terribly successful in my opinion.. but then some people can do that, I can't (except for very rare occasions).. I am very analytical.. Once I find something that invokes "something" internally that I want to express outwardly.. I'm set..