Isolating the keyboard from the videos you film

Hi,

Currently I'm using the crop feature in SeeMusic to isolate the actual keyboard and remove all the stuff around it, but I've noticed that some people have managed to have more of an empty dark space at the bottom that doesn't cut their hands off so much. I'm wondering how this is done while keeping the area around your hands dark (i.e. not showing knees, floor etc.) If anyone has any tips on the best way to do this I'd appreciate it.

Thanks

Ans