Find yourself a step, bench or any other contraption that you can rest a foot on, it needs to be about knee height.
Get into a forward lunge position with torso upright, core braced and hips square to your body, with your back foot elevated on the bench. Your leading leg should be half a metre or so in front of bench.
Lower until your front thigh is almost horizontal, keeping your knee in line with your foot. Don’t let your front knee travel beyond your toes.
Drive up through your front heel back to the starting position, again keeping your movements measured.