Go esay with power tools! You can screw things up in an awful hurry. That being said, I do use a belt sander to flatten the back to add overlays. I do mark reference lines, so I don't over do it. Maybe until you get more experience, do as Dana advised, and go with hand tools. Cut your back on a shallow angle, and your overlay material the same, and they should end up looking something like this:
Leave the overlay oversized, and work it down to final size by hand.
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