First, having the right form is key in this move as it is extremely easy to cheat yourself and compromise. Your legs and feet must be parallel and hip width apart. You take a giant step back/or forward (keeping your chest up and shoulders above the hips), bend both knees at the the same time as close to 90 degrees as you can. You can have hands on hips to start. Continue up and down like a carousel horse, or you can step back together and alternate backwards/ forwards stepping lunges. As you build up strength, you can add weights, and you can even work towards plyometric lunges for some added cardio.
Your core and lower back must be in good shape to hold your body upright. As a bonus, your calves will get some toning and your balance will challenged. Stay strong and focus on keeping your front knee over the ankle and your hips parallel. You can also raise the front heel to get both your calves burning and a greater challenge for your balance.
While lunges are tedious, they should become one of your go to exercises. You can add them in while walking your dog or pushing a stroller. They are also easy to do in your living room with no equipment needed.