I recently finished in an orchestral work called Sonolith. I sent a midi of it to  friend and he asked questions about how I decided on this how I decided on that. The truth of the matter is that it wasn't so much a decision, it was an intuition.
I've never been one to pre-plan. I did that once or twice when I was learning, very early on in my career. After that I always tried to be interactive with my work. By that I mean to listen to what I've done up to that point, and I listen in my mind, not at the keyboard. And then connect to the flow of the piece. So I'm continuing an improvisational line of thought. And in many respects my compositional technique is a kind of an improvisation.
That doesn't mean that I don't carry over any ideas from one day to the next, that I approach each new day empty-headed. At the end of every day I try to write something that points me in a good direction for exploration. A lot of times I'll be thinking over those ideas throughout the day, even sleeping on them, waking up with some idea of what to do. But more often I'll look at those notes that I made yesterday at the end of the day, I'll try to follow up some, and then basically throw them away.
If I try to follow those notes it's almost as though I'm trying to reconnect with an idea that's already gone. It's a reconstructive surgery. And that's not what composition is. Composition is creation, it isn't working from Blueprints and plans. It needs to be much more spontaneous in some respects. It's the picture of a wild mind at work.
Now I know many of my colleagues don't feel that way. Well if we all felt the same way maybe we we'd write the same music. And in that respect oh, it's very difficult to really talk about a finished product of music and make any real sense, having to Resort so much to analogy. It's even harder to talk about the creative process. But if someone wants to sit down and describe the creative process well, ee all do it our own ways. We do it any way that give us results. And all those ways other people work can lead to new insights and understandings. So even if someone does something in a way that you find to be anathema in some philosophical or aesthetic sense of yours, it's always something you can learn from. Even if it's a definition of what it is that you don't want to do.
But back to what I've been doing. I've come through a number of different styles and expressions, everything from  neo-impressionism to a very rigorous set theory usage, to an extended exploration of formal structures employing an  extended tonal language. That last section was prompted by something I read about Reger, one of my favorites and these days it seems a sadly neglected figure. When people asked him why his music was so complicated he said he was only doing what doing  what Bach did using the language of his own time. At that was the chromaticusm of mid to late Romantucism. So I did the same thing with atonal writing, specifically intervallic organization and with a reliance on the chromatic and octatonic collections.
WIt was a very prolific time. Lots of sonatas, lots of writing for solo violin, and a real understanding of the dramatic functions of formal structure.
Now however, something new. I can only speak of it in ways that seem almost "new agey". Each section is just about itself. It's just music. It's the thing without modifications. No adjectives, no modifiers, the thing as it is. And this is reflected in the name of some of my most recent pieces; Now, This, Is
But that's not to say there are no guiding principles. One, keep it fresh. If I can predict it too much then I may have become too formulaic and may rely too much on skill, references to traditional formal structures, or just simply stop thinking. Part of our training is to learn how to mine as much as possible out of his little as you've got. And so I can always go there  , to those learned skills, and then just assemble minutes worth of music on an idea. If it didn't work okay. It may not be revelatory, but it makes sense enough to keep your interest. But that's not what I'm looking to do. I'm looking to get rid of all the artifice, to just address the thing itself. If it's supposed to be a big tone then that's what I write. And if you can start predicting what's going to happen then you stop listening.
There is a spot n Sonolith where I do something that is very regular and I do it four times in a row. But in the larger structure of the piece this regularity itself become something that can't be predicted. Because nothing else is that regular.
So deciding on do I decided to do something  is not so much the the way to discuss it. It's more reacting to things. How does it affect me? Could I have seen it coming?  Does it make you sit up and listen, or keep you listening when something hasn't happened. You're right of course; this is very difficult to assess. As anyone who is composed knows, when you first listen to an idea you may notice something odd in it. But when you go back to find it you can't find it anymore, it's gone. And that's because now you understand the context of it. The audience gets to hear it only once. They needs to hear it, in context,  the first time. So you have to really be aware when you're listening to what your first reaction is.
I also found myself when I was writing Sonolith that sometimes I just start writing that day without trying to connect. I saw it was  not clearly connected or suggested by the previous days work but it was an interesting idea and I wanted to follow it up. And, I thought, if it doesn't work in this spot in may work somewhere else. And if it doesn't work for this piece I will always have it in mind for another. By the way having an idea in mind for another piece seldom works. And that's because  ideas fresh today becomes a story when you come back to it at a later date. Kind of like how you would relate what happened to you the day it occurred and how you would tell your friends about it two months later.
And the surprising thing about these ideas that seemed to occur without connection to what happened the day before is they were always surprisingly right. They always work beautifully. 
Now that was surprising enough that I even questioned myself. Am I being too uncritical, am I losing my ability to discern things. Am I getting senile, am I crazy. But on the other hand it may be that I'm just connecting in a very spontaneous way to the creative impulse. Taking dictation from the muse.
I've developed so many techniques to such a fine level, pat myself on the back, that I have discarded them. That took a bit of courage because it felt like a leap into the dark. But I also realized that if I continued to rely on such a well-developed set of techniques I would be continually repeating myself. And I haven't gotten to  this point in order to not go any further. So this was the next step; jettison everything and go with your gut.
And surprisingly so far I've enjoyed every bit of this exploration. It's taking me deeper into more exciting spaces. And anytime when I see that I am starting to revert as Barton Fink would say to an empty formalism I stop and realize that I need to reconnect with the wildness of creation and not try to tame it.
So, to answer the question of how did I do it, I did it with pen on paper. How did I decide? Actually I got out of the way of deciding. I heard it. It seemed right. And so I did it.
I used to quote a response that the composer Harrison Birtwistle gave in an interview about one of his works, I believe it was called Cascades. When asked how you compose work such as that he responded "well I try to find a chord I like and then I see what I can do with it".( The interviewer responded, "indeed".)
That does make a lot of sense, and it used to make sense to me, and it's still an important thing to learn as a composer. But actually it makes no sense to me anymore. It's almost  what does the idea reveal to me, what does it want me to do.
Years ago I had a note on my wall to direct me to the muse as I sat down to write in the morning. It said, "Not what you want to do. Not what music wants to so. What music want YOU to do.
 I guess finally I have to simply reply to the question how do I do it? Got me.