Assuming you're talking about sheeting, its very easy to bend balsa. First soak it with rubbing alcohol or ammonia (windex works well for this). Let the wood sit for a few minutes. You'll feel it start to be easier to flex. Don't rush it. Make sure the grain of the wood is running span-wise. Once you can easily flex it over the surface, use masking tape to hold it in place. Now allow the wood to dry (overnight is good). When its dry, it will retain the shape of the curve, and can be easily glued in place.

Brad