I make small pieces of music. Fx. a Guitar riff, a bass riff, a piano riff, some chords that fit, some small rhymes, a rythm, or just an idea.
Then i put them together. Pieces with the same feeling. I think about the structure. What is the song about? Should the chorus come 2 or 3 times?
Then i often make some kind of intro. Then the soli (Guitar or keyboard), and then the outro. Then i play the song on a terrible mic or something, and listen to it, andsee if there is something missing. Then i introduce it to my friends or band-mates.