I've heard time and time again of composers who say "It is better [for beginners] to know your form, and have a general idea of important chord progressions before you begin thinking seriously about melodies, basslines, etc."
How does one actually go about this process of roughly sketching out the structure of your piece beforehand? How do you decide how long phrases, sections, etc. should be... or should you allow them to expand dynamically according to your melodies as you write them without prematurely confining them to said predefined space? This way the melodies would feel much more natural, and the length of the piece (or number of measures) would adjust accordingly.
I apologize if this question seems trivial, but I am in the process of writing my first serious piece, and I think it would be extremely beneficial to have a goal in mind at pivotal moments. Conversely, if this is an overly ambitious, "good-in-theory" idea, then please don't hold back in telling me so.