Knee dragging should only be performed on a closed circuit course in a controlled environment and not on public streets. Always wear appropriate safety gear when operating a motorcycle.
Understand the reason for knee sliding. The purpose of dragging a knee is to indicate to the pilot the angle of motorcycle lean. At high speed sportbikes don't turn through turns, they carve through turns by leaning over in the direction of the turn. This lean is is induced by countersteering and is essentially a controlled fall over to either side. The gyroscopic nature of the rotating wheels and the traction of the tires allows the pilot to lean the motorcycle over through the turn and return it upright as the turn is completed.
Use correct body position. On the balls of your feet, torso bent over the tank, head tucked down and hands on the grips with a relaxed grip.
Learn to carve the turns at slower speeds by inducing countersteering.
Practice throwing your knee out to the same position through the turns, even if it doesn't touch the ground yet, just get in the habit of throwing the knee out there to the same position every time. This "same position" every time is important because touching down the knee is an indicator of lean angle, if your positioning isn't consistent you'll have no way to judge lean angle consistently.
Get some pictures of you in action. If a photographer is available and you can see some pictures of how you look you'll know how close you are to touching down. You might also pick up errors in body positioning that are contrary to safe efficient operation.
Pay attention to, and learn about, the traction. You're also learning about traction as you go through these exercises. Watch faster riders, study their body position, how they get their weight low to the inside of the turn. Knee out, torso shifted to the inside, head out from behind the screen near the edge of the front fairing. By now you might be suspecting the bike has more traction available than you're using, and if it hasn't slipped or crashed you'd be right. Continue becoming more comfortable with lean angle and body positioning. The whole idea here is to carve the turn with the minimum amount of motorcycle lean as necessary by getting your body weight positioned correctly.
Be consistent. Dragging a knee is an indicator of lean angle so consistency is key. The more comfortable you become leaning the motorcycle over the sooner you'll feel that first scrape on the sliders. Focus on not hanging off the motorcycle to an extreme as well as not leaning the bike over so far that traction is easily compromised.
Lastly, ask for tips and critique. If Control Riders or Instructors are available ask them to follow you a few laps and give feedback, they're there to help you achieve your goals.