The first thing you need to do is using a big towel to roll up the baby. SO you can handle the baby easily.
Second, you need to practice more for the hand cupping. You can practice on your thigh. Your hand is curl up as a cup, keep your wrist relaxed and let the hand drop freely. When you doing the percussion on your thigh, the deeling is no pain, the sound seem being loud and echo.
If you are confident enough, just do it on your baby.
One important you need to noted is that, don't doing any percussion on the back lumbar.