The way we do it in our band is that the guitarist comes up with a song idea and we jam it out. If it sounds like a song we'd like to work on, we practice playing it over and over. After a few practice sessions, I'm familiar enough with it and can feel free to improve the basic beat. We usually record our jam sessions so if I played a part that really fit the song well, I can take note of it and not forget about it.
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