Great question — answering as the UphillTech team:
Not an expert but from years of experience I would say between the hard part of the top of your foot and your toes. So approximately near the balls of your feet. Personally though, I would say just practice holding the ball in two hands and dropping in onto your feet, trying to kick it gently back up to your hands while generating backspin. I think once you can continuously get this backspin on in, you should have found the correct place to make contact with the ball. Also don't forget to practice this with both feet as it will improve your control/ touch, balance and feel for the ball.
Full answer by JosephRyan: https://sports.stackexchange.com/a/28515 (CC BY-SA 4.0).
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