I try and write a new song every week on the guitar. It might take several weeks thereafter to tweak it a bit, but the basic foundation/structure or layout is there. I write the music on the guitar first. Usually it's a melody or riff that I have in my head, or something that I've accidentally played on the guitar while not thinking - those are the best. Anyhow, once the seed of the idea is in my head, it's just a matter of working out the parts - what's the intro, the verse parts, the chorus, the bridge, if any, etc. I usually don't write any particular solo, but I know whether a bridge, an intro, a chorus, or a verse is gonna come after the solo. It takes time to work-out, but usually the flow of the song will dictate how to structure and where it should go verse, chorus, verse, chorus, or verse, verse, chorus, etc.

I listen to my favorite bands and their songs and break their songs down into parts played as intro, during verses, etc. That way, the song itself becomes a collection of pieces that are tied together nicely.

I always id the key I'm playing in and try and stick to the chords in the key of that major scale, but since I use root/fifth chords alot, I don't have to worry about minor chords and when to use.

I usually don't write words to my songs, but the tempo and riffs of my songs usually lend themselves very easily to lyrics. It wouldn't take a genius to compose lyrics and/or a melody that fit nicely into the song.

I'm a pure guitar nutcase so I don't consider a "complete" song as a song only with lyrics. As long as there exists the structure of the song, almost anything lyrically can be composed or adapted to it. Of course, like anything else, the music may have to give in some areas and take in others once the lyrics are written.

Just my take.