Go on all fours and engage your Abs (suck your bellybutton upward).
Lift one leg off the ground and extend at the hip and at the knee at the same time, so as to drive your foot backward until your leg is fully extended.
Pause slightly at the top, then reverse the motion slowly and repeat for the number of reps, then perform on the other side.
Form tips
Keep your Abs engaged and a posterior Pelvic tilt : do not let your back arch when kicking back. Arching the lower back makes the movement a lot easier to perform, but also a lot less effective. When done properly, you should feel your posterior upper thighs working.
Keep your torso and hips parallel with the ground: do not let the working side open upward.