Go to the fleet office. Hand over the notice to your chief pilot. Let him know whats in the envelope. Say thank you for a great time at Cathay but you have decided on leaving. No more - no less.
That way you are lying him and CX up their faces - just as they have been doing it to you for a long time. Nobody up on 3rd floor (or any other floor at CX City) cares about you or any other of their fellow pilots/employees. If they say they do - they are only lying once more.
Like somebody else said. Dont forget to take a basing first to get your pension transferred to your own account. A lot of guys have done that before you. At the end of the day - its your money, not Cathay's.
Wish you the best of luck on what I firmly believe is a sound and good decision.