Load custom firmware to pixhawk

Can anyone direct me to a non-deprecated document on how to load custom firmware to a pixhawk? Everything I find seems to be out of date. The only thing I have found is that you can still put the mission planner ‘layout’ to ‘advanced’


Just in case anyone else is wondering. I just switched over to QGroundControl and it was quiet easy to flash any ‘stable’ ‘developer’ or ‘custom’ versions of the APM firmware. I then jumped back over to mission planner and everything was dandy!