This is a single arm version of a bicep cable curl. Cable machines are great for helping you learn proper form while building muscle.
Attach a stirrup bar to a low cable pulley.
Stand of the side of the weight stack with your legs shoulder width apart and your knees slightly bent and your abs drawn in.
Grasp the stirrup in one hand with an palms facing up.
Lower the stirrup to the top of your thighs.
Keeping your elbows still, curl the stirrup up towards your chest.
Slowly lower the stirrup to the starting position.