There several ways to lunge on a BOSU ball. The most basic one would be to put one foot in the middle, to have the other foot out enough so that when you drop your knee, it's right inline with the rest of your hips, shoulders, all, your head, everything. So and that's all you have to do. You just come down and up in this nice lunge. You can add dumbbells to make it a little bit heavier.
To make it a little bit more difficult now, what you can do is you can lunge, step back, and then lunge again, and step back. So now you have to push with both legs. Instead of just using one leg, you're using everything to push off. So you're using your quads, your ham strings, your outer thighs and your inner thighs, just to push off. Not even because of the lunge.
A more advance version would be to start in front of the BOSU. Put your back leg on, and you're going to take it down and up. And now to really enhance it, you're gonna jump up a little bit. So you come down, you jump back. Not up but back necessarily. So you want to makes sure while you're coming up you're going back. I mean, by doing this you're working so many different muscles groups. Because you have to push off, and that alone works the entire leg that's in front. And then you're stabilizing with your back leg and using your ham string to bring you back up.