Could somebody tell, step by step, what is the most effective way to study a score, if my goal is to copy a composer's "language" as an exercise: to create a similar piece and to learn from the process?
Should I start by figuring out the harmonies? How about the form?
I ask this because I know that writing in the style of a given composer is a common exercise, but I don't know how to actually do it.
If all you want is to copy a score's texture and instrumentation and you do not care for the form at all, figure out the harmonies, melodies, use of motives, and instrumentation and do not bother analyzing the form. (I did that for one of my arrangements, "The Wayfarer and the Setting Star" - an arrangement of one Kirby-series musical theme in the style of another Kirby-series theme, the latter of which I was fortunate enough to find a full transcription of all instruments of half of it.)
If you care more about the form and phrase structure and care less about the instrumentation, analyze the form and phrase structure (and quite possibly harmonies and motive use) instead of the instrumentation. This tends to be the tactic I use when I want to compose piano pieces that are in the style of existing orchestral pieces (e.g. "Monarch's Glory", a march in the style of Edward Elgar's major-key Pomp and Circumstance marches).
If you want to copy everything from the instrumentation and texture through the harmonies and motive use to the form and phrase structure...well, you've gotta figure out everything.
...Should I start by figuring out the harmonies? How about the form?
Yes to both.
But, you would also look at rhythm, melody, metrical treatment, scale degrees, counterpoint and texture, accompaniment styles, instrumental genres, and on and on...
But to do this well goes beyond a step by step guide. If it were that easy, then all the great master composer works would amount to mere formula. At the very least you can't lump "composer" together. You wouldn't have one step by step method that would work for two composers like, for example, Handel and Bartok, or Bach and Satie.
You should also ask yourself how authentically you are trying to emulate a given composer. You can probably convince friends and family that something is "in the style of Mozart" by playing cadential harmonies with Alberti bass accompaniment and simple even rhythms... but you probably won't convince someone that way if they can tell the difference between Haydn and Mozart.
...I ask this because I know that writing in the style of a given composer is a common exercise...
I think it's more common to use a piece by a master composer as a template to practice composition, but that isn't necessarily an exercise in mimicking that composer's style. I think the important thing is to be aware of the difference.
For example, you could take a Haydn sonata, make a harmonic reduction, note where the cadences and structural sections begin and end, then fill in that skeleton with your own rhythms and melodic contours. The harmonic skeleton gives you a sort of "safe" formal structure to cover a certain amount of space - sort like the lines of a paint by number picture - your add rhythm and melody to make it unique. You started with Haydn, but the end result won't necessarily sound like Haydn.
