In order to do an arm curl or a bicep curl and a tricep press, it's pretty simple. You just step onto the BOSU ball. Make sure you're in the middle of it, and then you're going to bring your arms up into a curl and then back down. You want to make sure your elbows stay by your side so you're not using any momentum.
And then also, you want to make sure that your knees are slightly bent. You don't want to lock them out on a BOSU ball. So you're just holding yourself up, taking it into a bicep curl.
And then for a tricep extension, you take one weight or two, you bring them behind your head. Again, you want to keep your knees slightly bent. You're going to tuck your hips under so that you're not arching, and then you bring your elbows down, and back up. So you want to bring your arms down, just at the elbow you bend.
And then to make this move more advanced, you can do all of that on the other side of the ball. So you flip the ball over, holding the weights before you get on. You step onto the middle and then the other foot comes on and you widen your stance a little bit, keeping your knees bent. And then again, keeping your elbows in, you curl.
For the tricep version you take your arms above your head and bend at the elbow. And then once you're done with that you can just hop off by dipping to the side and stepping off.
And that would be an arm curl or a tricep press on the BOSU ball.