A Bosu ball is great for lunges. You can do back or forward or combine the two. So for the backward lunge you would put one foot on, you would bring your foot up and then step back and lunge. Bring your foot up, step back and lunge. Just like that.
You want to make sure that your posture is good. Your shoulders are back and your back is straight. You want to make sure when you lunge that your knee is going towards the floor and is right under your hips.
Then, in order to do a front lunge it would be the opposite. So you would put one leg on it, bring your toe on it, so you don't want to put your whole foot on it. And you want to make sure that when you go down your knee does not past your ankle. So right now I'm not far enough up, so watch what happens. My knee goes way forward. But if I take another step forward then I can just go down, straight up and down.
Then if you want to combine the two together, you can put one foot on, you lunge, so this would be the back one. You lift up, and you go all the way through to the front. Then you come back, all the way to the back.
And then if you want to make it a little bit more intense, you can add weight. So you have your weights, you have your back lunge. You're lunging, bringing it up, or you have your front lunge. Again, you're way forward, down, up, down.