My guess is grab and pull? I'd say the thumb and index finger would get you the most grip, but it's possible that all 5 fingers could be used. That seems like the most simple explanation I can give. I don't really think there is a best way to do. It's a pretty simple thing and doesn't require much mechanics.

It depends on the girls you're doing this to as well. Some of us are more sensitive down there than others. You might start with light tugging at first and guage her reaction and then gradually make it harder as you go.

If it's your sub that you're doing it to, you might also try clamps and weights to get her used to it... as long as she's standing, this method should work to get her used to the tug and pressure.