Set FLAP_IN_CHANNEL to whatever channel you want to control the amount of flaps. Do not do any mixing on the radio; let ardupilot do that.
Then set your servo_functions to 24 or 25 (and potentially servo_reversals) depending on how your aircraft responds in FBWA/Manual mode and when you command flaps via your transmitter.
Also consider reading/asking questions in this related thread: Landing Flaps Parameter Questions
Good luck!