To do a pelvic tilt on a BOSU-ball, what you want to do is you want to bring your heels onto the middle of the BOSU. You're going to keep your butt maybe two inches away from it. Lie back on your back. Keeping your hands by your side, head down. Just look up at the ceiling and you're just going to lift your butt up and then down. Now when you lift up, you want to squeeze your butt together and down. To make it even more challenging you can squeeze your inner thighs together or even put a ball in between your inner thighs and squeeze that.
So you're just going to lift and squeeze and then back down. Lift and squeeze, back down. Another more advanced move is to lift one leg up. So you can have one leg, keeping both knees together up in the air, and then you lift and you lower. You want to get yourself all the way up, so that your hips are in line with the rest of your body, and lower. You will definitely feel this in your hamstrings right away, especially if you're doing it one leg at a time. Then at the end if you want to finish it off, you can hold it up for maybe 30 seconds, come down.