Let me show you personally how I do it, and maybe this will give you some ideas.
What I do might seem awkward and rather intense for some, but I think it really brings out honest sounds. I first envision the sound in my head (what mood I am in usually determines the sound). If I were you I would feel out a chord I like and then build your measure around it.
I advise against making your melody and building harmonies around it. Obviously people do it and I do it too when I'm lazy and it does sound good sometimes. But I believe "Melody and harmony go hand in hand" so they should be made at the same time

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Obviously this wasn't a deep tutorial but I hope it helps somewhat