On camera? I'm not sure there's a whole lot to it really, Patrick.
This^A speedlight, yes, but there is no need to diffuse it. You're not looking to add enough flash to be obvious so diffusion isn't necessary.
Patrick, just go out and play with it in TTL.
What Dean said.
IIRC If you shoot Nikon, the ambient and flash balance to create an exposure. So if you want the flash as fill, you might need to overexpose the ambient slightly (it all seems weird to me).
With Canon it's easier, you can shoot your normal ambient exposure, then dial in -1/2 stop on the flash, if it's too bright, dial it down, etc.