Queiscen has left some really great posts that are very similar to what works for me. I used root chords and started playing around to see what I did and didn't like. By using the notes in each specific chord I learned how to string together different tunes. I made sure my timing was constant, and though I rarely write anything down but the chord, I have been able to write many songs. I actually prefer vocals, and write to suit my own styles and needs. By using guitar chords at the top of most pieces of music on the radio I can play just about anything.
I really have to see about getting the chart I have on here for people to use if they do so desire. Check out the the posts from quiescen, he offers a free lesson at the end of each of his posts. They're online.
I hope this helps.
~B~