You may do this with a free weight in one hand to challenge stability as well as build strength. Have the DB or KB in the hand on the same side as the working leg. Place the other arm on a wall/anything that will give you some stability.

You may do this with a smith machine and go heavier if the goal is hypertrophy (Muscle growth) Important to go slow and focus on pushing through the heel of the working leg. Only let your toe of your back leg LIGHTLY touch the ground. If you need to push off the ground with that back foot–you engage your quad. If you find it impossible to not push of the back leg, lower the amount of load (weight)