There are a lot of standard answers you'll receive to this question. E.g., Solidify your harmony and counterpoint skills, then study form from books x & y. The problem is that all of these texts are style dependent. In order to learn the art of composition with a broader scope, you need to consult something that isn't biased to suit a particular style, whether that be Baroque counterpoint, Romantic harmony, or high-classical form.There is an excellent composer and teach who has written such books, which are free of charge on the internet. His name is Alan Belkin, and he teaches at L'Université de Montréal. The texts are in multiple languages, including English. I promise you that this is your best resource outside of private lessons. Best of luck!https://alanbelkinmusic.com/ABWritingNAV.html