The best way, presuming that you do not have any contacts, is to visit courtrooms having OS determination & identify Counsels who you think argue well (whether or not they are a Silk), following which do some background research on their work and then directly approach them. Why do I say this? Formal applications are rarely called for; candidly, pupillages at Cal HC are mostly through recommendations. The realistic shot is to genuinely appreciate the Counsel's work & expertise, do some meticulous research on his/her work and then politely approach the Counsel in court; however, it's a high-risk, high-reward situation – at worst you would be castigated on the tenets of decorum, at best you will walk out with a juniorship in hand. I can tell you most assuredly that this is the most effective approach, despite the potential risks involved. However, do not resort to outright flattery; it can be easily pierced through. Be thoroughly prepared, a 2 min conversation can do wonders if done right, and contrary to the popular rhetoric pertaining to the profession, people still can turn out to be extremely benevolent if they see a semblance of promise in you. All the best!