Models

I am just wondering, how do you use a model to follow when writing? Of course, one wants to develop one's own voice and not imitate another composer. But, in the beginning stages, I believe it is acceptable to use models when writing. Now, when using a model, do you just follow the form? Do you use the same melodies? Do you just use the same tempos, keys, etc.? What are the steps you take to use a piece as a model?

Also, can anyone here suggest a few good pieces to use as models for a piano sonata, a symphony, and a piano concerto. When I look for models, I usually look for a "standard" work. One that follows the standard form, style. Also, I AM LOOKING FOR CLASSICAL MODELS. Not Prokofiev, not Tchaikovsky. Maybe some Haydn, Mozart, or Beethoven. Remember, I look for a standard or normal symphony. That would not include Beethoven's symphonies 3,5,9, or something like Haydn's suprise, etc.

I excluded Beethoven Symphonies 3 and 5, because, for symphonies, I was looking for a Haydn style/form symphony. I have had a couple of past teachers tell me (on the topic of composition; they were piano teachers) that Haydn's Symphonies and sonatas were the most "classical style" and very formal and standard, so to speak. So, I was looking for a piece in Haydn's style such as Haydn 97, Beethoven 1, etc. I think that Beethoven 3, though in sonata form, is more of a "Beethoven creation", with the long coda in the 1st movement and the introduction. In Beethoven 5, the attacc a a between mvts 3 and 4, and the return of the 3rd movement in the 4th movement, really just doesn't seem to me like a "Haydn-like, formal, standard" symphony. I don't know if I'm thinking the correct way, but as I have never formally studied composition. I've had a few theory teachers and read many books, but never from cover to cover.Thanks!