Start standing straight. Bend your knees in a kinda-sitting position (not fully though). Extend one leg and lift knee upward (pointed the toes), return to an extended position, bring leg back to the other one and straighten up the legs. It is important to perform the exercise correctly to get the best result, so avoid swinging the leg. After doing series for one leg, switch them. (Working on one leg will make you feel the muscles burn :D) Start with 10 reps and built it up.
