i will answer these questions to the best i can, some of these are ones i think i can handle well! sorry for the essays, but i hope this advice helps u as much as it's helped me over this year
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ZAALAN
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A• To begin, I find "creative juices" are just experiences filtered by your unique, perceptive brain to be then utilized within expression in your art. The mind's potential to understand itself is always there, and creation of art is a strong execution of this. Ideas are floating around in your mind, and you must understand how to snatch them so you can utilize their potential.
Your creativity is a general thing. Music can be inspired by non-musical things just like how the inverse is true as well. Thus, creating in other rewarding fields is beneficial too. Let's cook! Food is meant to benefit you and be enjoyed, and cooking it yourself gives you a live deconstruction of what makes the meal. I don't feel it immediately, but I find it inspires me to see this deconstruction of an idea that I can afterwards enjoy, also. Eggs do work well for this since you can cook them in so many ways and utilize them in almost anything! I like to make many forms of eggs from scrambled to poached and putting various things on there, especially salsa. If you can, you want to go for Ms.Renfro's (Habanero!) because it's the best. Writing music gets easier for you when you 'write' in other practices too, such as this. It can be writing, painting, or whatever else -- but food is a daily necessity and I like to eat, so that's my top choice.
Environment also matters, and one you're unsatisfied or indifferent about will reduce your creativity. If you have a messy room, clean it up. Have things in there you don't care for or never use? Store them elsewhere, or throw them out. If you like visual art, put such pieces that you like on your wall, but only a few. You want your main area that you're in to feel like your space to relax and let thoughts simmer, not be distracted nor over-stimulated. Just be content with the room you make music in and/or your bedroom, or it will distract your workflow and thoughts, reducing the sense of satisfaction with self and overall confidence.
Now, while the past two sources of inspiration have seemed influential in non-musical ways, you can still draw a ton of life from music itself. It's important to study music, especially pieces that you really enjoy and want to know deeply. If you don't know theory, look into it. There is a separation you must have between listening to music, and studying music. If you just absorb the music on the surface level, listening, you're going to approach your music with a mind polluted with ideas that you can't fully understand, but only approximate at. On the other hand, studying music gives you an understanding because you aim to learn more than what only listening can accomplish. This is just as productive as writing music, and you can do it by learning theory, or covering a song you feel you must know more about. If you don't understand a song that you want to understand, deconstruct it until you do. This is something you will gain from, expanding your intuitive abilities far more than casually listening or writing forever will.
When you listen to music, it's to enjoy it. When you study music, it's so you can apply it.
Long story short, make your own foods when you can, exist in a comfortable environment, and study music. Your ideas are always there within potential, and you just need your mind to be stimulated enough to access that.
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B• (the other ppl gave great advice on this)
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C• I can be brief with this and say that my own skill-level with melodies is getting progressively better; but why? Up until recently, I'd write chords first in a song, the progression usually going: chords > drums > bass > melody
Now, the melody is what carries a song forward, and is the strongest expression of the statement the piece as a whole is bringing. Therefore, I've found that writing a melody first is best then bass, chords, and drums/whatever else you have. The bass is a low, rhythmic counter-melody that fits with your main melodic line. Chords should flow with these two elements while still voicing their own purpose. Percussion should emphasize what is meant to be impactful in the rhythm without the perc there, so putting kicks where strong chords are helps a lot. But overall, no part should be copying what another is doing, they should all be expressing what their own variations on one idea. Oh, and one more tip: the song should sound 'complete' in composition with only the melody and bass. Bass carries the counter-melody and rhythm, the melody expresses the main point of the song. If it isn't strong with the melody and bass isolated, you still need practice writing those elements, because in the end those are the core of a song.
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GOLUIGI
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(iunno may write something here later, i'm not the best source on that kind of stuff especially when trying to explain it!)