Practice is the main lesson, but here are a few other ideas:
-Start with a slightly deflated ball
-Using your hands, drop the ball to your foot and use your laces to tap the ball back to the hands. I believe the ball should make contact at the point the knee locks out.
-Alternate both feet and mix in the knees.

Keep at it and keep it fun. Try to add to your last record. Later you can mix it up: foot, foot, knee, knee, head, foot, foot, knee, etc... Once you can string a few together, get a friend to play with you and try to better each other.

concentrate and focus on how you kick the ball