I have found that composing music is an ongoing personal process, rather than a universal learned skill. For example, some people find it useful to compose every day, whereas others only speak when they have something that must be said. Some composers enjoy short, song forms, whereas others are interested in longer forms of music. Some enjoy composing for chamber music, others large groups or the orchestra. I think the first step in cultivating your process of composing is to think about what you like to listen to, and what you feel composed to write.
Listen to a diversity, and as much of music as possible. Find what you are drawn to in combination, form, historical context, and style. Do you love ancient music? Are you most at home with surrealist composition? Is contemporary jazz, or free improvisation calling you? Find what music turns you on the most.
Identify composers whose music and philosophy resonate with you. Read their writings and listen to their interviews. Get to know who you feel closest to and study their process and their composition. Don’t hesitate to contact this composer and ask if they are willing to take you on as a private student.
Read as much as possible about composition and creative process. There are many books on these topics. I have found that reading books by poets, painters, architects, and writers are filled with gems for the composer. Art making is art making, look in unexpected, as well as expected places.
Studying theory and analysis is very helpful in learning textures, colors, forms, and rhythmic settings that you might not have known. Studying music theory of other cultures is also very useful for fresh ways of thinking.
Look for connections, rather than differences. Observe rather than judge.
Read biographies and memoirs of creative artists. Read theoretical works by continental philosophers such as Kant, Schopenhauer, Nietzsche, and Heidegger. Read Carl Jung on active imagination and On The Relation of Analytical Psychology to Poetry.
There are a few, basic, compositional techniques for thematic development. These were all outlined and illustrated by Arnold Schoenberg in his text, Fundamentals of Musical Composition. This one text can really teach you all of the techniques in thematic development.
Try to “see” audial stimuli and “hear” visual stimuli.
Talk with others who compose. Get together with other musicians and ask a lot of questions about their who, what, when, why, & how of composition.
Spend time walking or sitting with yourself, quietly contemplating your own who, what, why, when, & how.