Conceiving the entire form before beginning a composition

I recently bought Fundamentals of Musical Composition by Arnold Schoenberg and I am excited to be reading it as it is the first real reading I am doing on musical composition. In the first chapter he writes the following:

"A composer does not, of course, add bit by bit, as a child does in building with wooden blocks. He conceives the entire composition as a spontaneous vision. Then he proceeds, like Michelangelo who chiselled his Moses out of the marble without sketches, complete in every detail, thus directly forming his material.

"No beginner is capable of envisaging a composition in its entirety; hence he must proceed gradually, from the simpler to the more complex. Simplified practice forms, which do not always correspond to art forms, help a student acquire a sense of form and a knowledge of the essentials of construction. It will be useful to start by building musical blocks and connecting them intelligently."

My question is how a composer can in their vision of the composition, imagine the entirety of the form before composing even a single motive. Sure, before beginning a composition, the composer feels the ineffable essence of what they wish to express in their composition, but the form is part of that too?

I was surprised to read that a composer does not add bit by bit, since it seems natural to compose by writing a theme, and then a contrasting one. I have not, though, had much success doing so, so I guess I must begin small and internalize form as a begin composing small forms for practice. I may be misunderstanding something, but reading this seemed like a reality check that I was thinking of form wrong as a composer. Or maybe this "spontaneous vision" of the form is something only the greatest of the great composers can master. I don't know; I'm a beginner.

Thinking, "I should write a theme first and then a contrasting one," is exactly the sort of thinking about form that he is discussing.  You sound like you are well on your way to thinking about the large scale issues with a piece, rather than just writing note by note.  

It sounds like he's talking about how beginning composers just sort of work from one note to the next without any rhyme or reason.  There may be no theme at all, just a random succession of notes that they liked for five minutes straight.  

Everyone works one note to the next, but the more experience you have, the more you'll be thinking ahead about how to organize yourself in the choosing of those notes.  Thinking, I have a fast section, so next I should have a slow one for contrast... that sort of thing.  Or, I'm going to have range issues if I give this bit to that instrument without modulating.  Do I want to modulate, or do I want to give it to a different instrument...  

And he wants to suggest some simpler exercises to get you started thinking about these sorts of things before you decide to write a whole symphony.  Use fewer instruments.  Write a short theme and see if you can turn it upside down.  Practice writing a fugue.  

Also, Schoenberg is writing in a way that makes him sound like a big smarty pants so he can scare you into doing your homework, so don't be intimidated into a state of confusion, just do the exercises.  

(My best guess at what he was going for at any rate).