Few tips: Videos are not just visual but also got audio, remember that. You want him to be able to hear you as well.
As for what to do? Depends on what he is into / what he wants. Is your only task to masturbate? Are you meant to film yourself cumming? Think about what you want to wear, where you want to be filming yourself. Do you want to use toys or just your fingers? Do you want to shove them inside or just touch your clit? There are lots of different ways you can do simple masturbation. And that suggests he is not asking for anything more kinky...
Best guess is to just ask him if he has wishes about it and go from there.