Writing has many levels, there's the musical notation/chords/voicings... there's the vocals/or not....there's the rythms/feel....and the arraingment. Each require attention and each should support your musical idea (when finished).
As a drummer (who fakes the other instruments) I find inspiration for a new piece in various ways. Sometimes I write the piece with a specific time signature(s) in mind, such as starting in 5 and playing with a drum feel. Sometimes I can hear a bass line and I'll mumble it as I play, sometimes I lay down a rythm and play bass after the fact. If something crawls out that is musical I usually re cut the drums to fit. A chicken and egg thing. A few itterations of that and you may have the core idea on it's way.
To me good music has tension and resolve (like a good movie), this can be done with chords/voicings and/or with rythms as well (this is where a keen ear and study of music pays off.... analize your favorite music for these often hidden tricks).
Rythmic tension may be an odd meter and resolve may be even, ...you could write a verse in 5 and the chourus in 4 or 6 as an example or a 7 that moves to an 8. Keep it simple, these changes should have musical meaning within the piece.
Seldom do I invision an entire piece/song, often it's a slow evolution and usually (if you record often) you may have that missing piece/hook in another half baked project on your hardrive just waiting to be mined (record everything/record often). This type of Frankenstein music creation is not uncommon especially in prog where pieces are longer and more complex in general.
I also like to take a part and play it to death, stretch it out and let it find itself, you may end up twisting it inside out and finding a better way to play it. It's good to have a live mic incase you hear a melody in your head you can mumble it. Allow the music to come out on it's own. Again, this is where recording everything helps. You can go back (a few days later with clear ears) and analize what you captured, disect what worked from the crap.
And it's probably best to work with someone else along the way (even Pat metheny needs Lyle Mayes).
did I bore you to tears yet?