Auto Land is set by the mission control scripting only. Throttle and
altitude is controlled by the autopilot. After getting closer
LAND_FLARE_ALT meters from the target altitude or
seconds from the target landing point the plane will “flare” to the
LAND_PITCH_CD pitch (in centidegrees) and will hold heading for the
Setting up Plane for reliable auto-takeoff and landing is very airframe dependent, and it is recommended that you first get some experience flying your aircraft in FBWA mode, and be ready to take over control in manual or FBWA mode the first few times you use an automatic takeoff or landing.
You should also look through the complete list of parameters, as there are a lot of parameters that help control takeoff and landing for different situations.