How to setup a transmitter to control ardupilot mega modes (mine is Optic 6)

For me, it was rather difficult to set up a switch on my transmitter to change mode on the APM.

I finally understood but I think it can be explained a little more in details. 

First of all, here's a list of link adressing the issue

• http://www.rcgroups.com/forums/showthread.php?t=1477169
• http://rangevideo.com/forum/showthread.php?1994-Auxiliary-channel-on-Hitec-Optic-6-radio-This-is-my-method
• http://code.google.com/p/arducopter/wiki/AC2_ModeSwitch