Great thing about a BOSU is you have to stabilize just to hold yourself up. So doing a plank on a BOSU is the next level to doing a plank on the floor. So by doing one on the BOSU you can put your hands down. You want to make sure that they're both equal distance apart in the middle of the BOSU, shoulder width apart, holding yourself up.
Now this might be difficult on someone's wrists, and if you do have wrist issues the best thing to do is to instead do this on your elbows. So you can either come down like this, or you can start on your knees, put your elbows down, and then push yourself up. And you're going to keep your back flat. You want to squeeze your shoulder blades together. Keep your neck in line with your back. And maybe 20 to 30 seconds would be good. More advanced, maybe a minute.
And then the other way you can do a plank on the BOSU ball is by flipping it over. And again, you can either hold it up on your hands, but if you have wrist issues, then you can drop it to your knees, put both elbows down, and then hold yourself up.
And then if you want to get playful with it, you can bring one leg up, or you could bring the other leg up, or you can do jacks while you're down here.
All these are different versions of planks on a BOSU ball.