It's actually quite simple once you realize what you're supposed to do:

1. Flicking the joystick, rather than pressing down on a direction.
   
2. Hitting R Trigger before flicking.

It takes some practice, but you'll eventually be able to kick like nobody's business!

Hopefully, this guide helps at least one unfortunate soul to avoid going hollow because you accidentally kicked Pontiff Sulyvahn instead of dealing the finishing blow .