Bend your clients knee so that their foot is flat on the bed. Stand in from of your client and bring the wax over the top of the knee down to where you would have already removed the hair on the leg. If you're right handed, stretch the clients thigh with your left hand so that you can scrape the wax close to the skin. Get in a rhythm and don't stop until you have covered the knee in the thin layer of strip wax.
Then take a clean strip and stretching the thigh again to make the skin taut so that's it's less painful, start removing the wax and hair from one side of the knee to the other. Then remove from the top half of the knee. Never put your strip over the bend of the knee.
Finish with oil so they don't feel sticky.