Most of the time, when I write, the guitar part comes from an idea I got by just jamming out and playing around in the past. Then when lyrics start flowing out of me, which usually only happens when something dramatic or emotional happens, if I have a story of an adventure I went on or something. I fit them to riffs I've figured out on accident. Then once I make a melody for lyrics that fit with the main guitar riff, I follow lyrics, and figure out the chords to fit them for the rest of the song..... if that makes any sence.