It's usually easy, sometimes I think it's more a question of how not to do it that's harder to answer...

If it's out in public subtle things can be enough. Like smiling, and accidentally showing some flesh, or touching.

In private there are many many ways. I think you can picure many. But the most direct is of course to strip for him and show him how much you are aroused by him. Don't be overpowering so you scare him. Most guys get a big turn off if they feel like they are losing control and the girl is to pushy, so make yourself cute, sexy! All depends of your relation to him!