How to compose a symphony? This discussion doesn’t deserve a comment, but a book, a huge book! But I will try to be synthetic.
There are fundamental aspects: 1- You may know what is your style. A Haydn symphony is very different from a Mahler symphony. But the true is that since Beethoven, compose a symphony is one of the highest stages of the music, like Masses, Oratorias and Operas! 2- Then as Austenite recommended you, is very important to listen the great contributions made by the great genius, I give you twenty examples which I consider fundamental. - Mozart 39, 40 and 41; - Haydn at least the104; - Beethoven 3, 7 and 9. - Schubert 8 and 9 - Brahms 3 and 4. - Bruckner 4, 6, 7 and 9. - Tchaikovsky 6. - Mahler 5, 6 and 9. - Rachmaninoff 2. 3- Gustav Mahler once said that create a symphony is create a new world. I ask you if you are prepared to create a world? It is important first to have composed piano sonatas, then after string quartets are also a great step and help. 4- Then learn orchestration in one (or more, more is always best) treaty, I recommend Rimsky Korsakov.
These were the “pre-steps”. Now, knowing what you want:
5- First do the structure and the main themes. 6- After you start from the beginning composing on the piano, still not very worried with orchestration. But always thinking that it is an orchestra work and not a piano work (could be impossible to play for a pianist), it is good and normal that you start thinking in orchestration aspects, but what I want to say is that in this phase it could not be your first problem. 7- Then after have the piano score orchestrate. I give the advice to not use computer programs, use your ear!
Don’t forget good themes, good harmonies and good counterpoint! Best wishes.