The FBWB mode is similar to FBWA, but ArduPlane will try to hold altitude as well. Roll control is the same as FBWA, and altitude is controlled using the elevator. The target airspeed is controlled using the throttle.
To control your altitude in FBWB mode you use the elevator to ask for a change in altitude. If you leave the elevator centred then ArduPlane will try to hold the current altitude. As you move the elevator ArduPlane will try to gain or lose altitude in proportion to how far you move the elevator. How much altitude it tries to gain for full elevator deflection depends on the FBWB_CLIMB_RATE parameter, which defaults to 2 meters/second. Note that 2 m/s is quite a slow change, so many users will want to raise FBWB_CLIMB_RATE to a higher value to make the altitude change more responsive.
Whether you need to pull back on the elevator stick or push forward to climb depends on the setting of the FBWB_ELEV_REV parameter. The default is for pulling back on the elevator to cause the plane to climb. This corresponds to the normal response direction for a RC model. If you are more comfortable with the reverse you can set FBWB_ELEV_REV to 1 and the elevator will be reversed in FBWB mode.
Note that the elevator stick does not control pitch, it controls target altitude. The amount of pitch that will be used to achieve the requested climb or descent rate depends on your TECS tuning settings, but in general the autopilot will try to hold the plane fairly level in pitch, and will primarily climb or descend by raising or lowering the throttle. This can be disconcerting for people used to flying in FBWA mode, where you have much more direct control over pitch.
If you have an airspeed sensor then the throttle will control the target airspeed in the range ARSPD_FBW_MIN to ARSPD_FBW_MAX. If throttle is minimum then the plane will try to fly at ARSPD_FBW_MIN. If it is maximum it will try to fly at ARSPD_FBW_MAX.
If you don’t have an airspeed sensor then the throttle will set the target throttle of the plane, and ArduPlane will adjust the throttle around that setting to achieve the desired altitude hold. The throttle stick can be used to push the target throttle up beyond what it calculates is needed, to fly faster.
As with FBWA, the rudder is under a combination of manual control and auto control for turn coordination.
You should also have a look at CRUISE mode, as it is generally better than FBWB, especially if there is significant wind. In CRUISE mode the aircraft will hold a ground track as opposed to just levelling the wings when you don’t input any roll with the aileron stick.