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).