I start out with a title. This helps keep me focused on the subject. Before I ever grab my guitar, I get the lyric finished first. That way, the lyric doesn't become laminated to the melody when it comes time for the rewrite phase. Or, grab your guitar, or sit at the piano first and compose your melody, then when the melody is just the way you want it, write you lyric to fit the melody. I've done it both ways. There is a book about the craft of lyric writing by Sheila Davis I highly recommend "The Craft of Lyric Writing" In fact she has two or three books out on the subject. Even if you just write songs for the fun of it, the money is well spent for her books. Helped me out a lot steering me through the intricacies and clear of the pitfalls.