the way I have written music, is firstly, start with what sort of style of song do I want to make. ill start by saying to myself, ok fast rock song, or funky song, or ambient etc... This way you know what style of music your going to be writing.
I feel personally, when I write music from a chord progression, the music really starts to become music for a band. So because I use software based instruments ie, bass drums, keyboards etc.. i usually use my guitar to write the bass lines and then drums. because drums usually sets the tone of the entire song. Drums tells you how fast the song will be and what feel it will have.
In the end its different for everyone, and thats the beauty of making music. Sometime I set out to write a certain style thats playing in my head, and by the time I lift my head from my PC, i have written a tottally different song. usually a melody catches my imagination and it just grows from there.
I once heard a novel writer say that he never plans out a book, as that would be a nightmare. he starts with an idea and it just grows from there.
So find your self an idea ie, a melody or a bassline or a drum beat that you like, and let it grow from that.
If you like listen to my tracks on my myspace.