After 35 years of playing music, I find that I can compose a lot of music by playing or soloing to a backing track and recording on the spot. Don't worry about making mistakes or not knowing what to play at first. You need to experiment and record, even if the result initially seems subpar. After recording for 5 or 10 minutes, stop and listen. I bet you'll find at least a couple of notes that ended up sounding cool. When you find them, keep them, own them, memorize them, and discard the unwanted portions. Then repeat the process. Before you know it, you'll have a collection of riffs that you've created, which you can start sewing together. Knowing music theory significantly enhances your writing skills. There are numerous tutorials on YouTube, but I recommend learning scales and modes and practicing with backing tracks. Recording your music and using a Digital Audio Workstation (DAW) will greatly enhance your overall ability to create.
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