For a Backside ShuvitWhen you jump, you want to push the tail of your board back behind you, using the ball of your foot. Your front foot should stay just above the board, so it keeps the board from popping into the air. Some people like to also kick with the heel of their front foot, to help it spin more - this only works if your foot is NOT in the center of the board, which is where you want to have it once you have the Shuvit figured out. I prefer to just get that front foot out of the way and let the board spin (but keep your front foot above the board - don't pull it too much out of the way). In a Shuvit, your back foot does most of the work.
For a Frontside ShuvitWhen you jump, you want to kick the tail of your board out in front of you. Otherwise, everything that works for the Backside Shuvit works for the Frontside Shuvit. Your front foot should stay just above the board, so it keeps the board from popping into the air.