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With regard to the Ethiopian crash of the 737 Max 8 aircraft, where according to preliminary reports, it appears to be related to the MCAS which is a safety system that pushes the nose down in the even of an excessively high angle of attack (pitch attitude), how difficult is it for the pilots to simply disable the MCAS and manually fly the plane?

To clarify my question, what I'm asking is can the pilot disengage the MCAS and if so, how do they do that? Is it just flipping a switch or popping a breaker, or is it a complicated set of procedures?

MCAS doesn't have its own on/off switch

It is a fly-by-wire feature designed to account for a particular flight regime that would not (or was not expected to) be encountered very often in normal operations, and is intended to account for some of the aerodynamic effects of the LEAP-1B (CFM International) engine installation for this model. Its activation requires a number of preconditions, but we'll get to that in a moment.

A few things that should disable it (with caveats)

1. Lower the flaps. It is intended to work only if the flaps are up.



2. 
   

   Turn the Stab Trim switches to OFF. This disables the horizontal stabilizer's trim completely, and reverts to manual trim (there are two guarded stabilizer trim switches in the aisle stand, see Windshear's answer). This means that the pilots must move/rotate the trim wheels in order to apply pitch trim during flight.
   

   
   
   

   The manual pitch trim appears to be what a few crews did prior to the LionAir crash in October 2018. It is unclear how many of the crews knew that is was MCAS, versus any other trim or pitch anomaly. The previous LionAir crews on the accident aircraft ended up flying to their destination manually. (Original source i the Preliminary Report from that accident).

   
   
   

   
   

   ... pilots encountered problems involving the AoA as well as the pitot tube used to measure airspeed. In a flight in the same plane the day before, to Jakarta, the pilot experienced many of the same symptoms as the pilots on flight JT610: the stick shaker activated during rotation, an indicated airspeed warning alert appeared, and the aircraft began automatically pushing the aircraft nose down.

   
   
   

   The pilot, after determining that his flight display system was malfunctioning, ran a runaway stabilizer non-normal checklist which led to the MCAS being disconnected when the stabilizer trim switches were turned off. The copilot flew the rest of the flight using manual controls and without autopilot.

   
   
   

   That Jakarta flight was using an angle of attack sensor that had been replaced after the previous Lion Air flight to Denpasar experienced problems. snip However, it is not clear whether the pilot communicated that he ran a runaway stabilizer non-normal checklist during the flight, which might have alerted the airline’s engineering staff that there was still a problem. (source)

   
   

   
   


3. 
   

   Enable autopilot. It is supposed to only work if the autopilot is off. But...CAVEAT

   
   
   

   That last part is a little bit complicated: the autopilot may not stay on if it - the autopilot system - keeps getting spurious signals from the AoA sensors, or if the pitot static system is providing bad data to the FCCs.

   
   

How do you know that you need to disable MCAS?

A more apt question might be "when do you know that it's time to turn off Stab Trim switches?"

The problem for a given crew: how do you know what sub-system that is giving you trouble? The AD released in December of 2018 updated the Runaway Stabilizer procedure. Since there are other malfunctions possible in the trim system for pitch, such as a trim runaway, the same symptom may manifest during malfunctions with different causes. (See the example above). If additional symptoms are present, such as a stick shaker activating from an erroneous AoA indication or some other fault, which problem do you solve first?

Which problem are you having?

Are you having both problems, or even something else?

That concern, the relationship between different potential failure modes, gets to the heart of

1. What is in the pilots manuals?




2. What training the crews do, or do not, get on this sub-system?

The MCAS is only supposed to activate under certain conditions (in theory): high angle-of-attack, flaps-up, flight with autopilot off. This includes high angle of bank flight in such conditions.

CAVEAT

Take a look at that last bullet in the lower right of the figure. It's only partially correct. When the pilot overrides with yoke trim, MCAS will try again five seconds later (after the pilot has stopped) unless the triggering condition, or signal, goes away.

"In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds," Boeing explained to operators in a bulletin issued last November, following the JT610 accident. "The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 sec. after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down
stabilizer continue to occur unless the stabilizer trim system is deactivated." (source)

It is this system behavior that is believed to have ultimately overcome the best efforts of the pilots in the LionAir crash. They were airborne for about six minutes before they were no longer able to overcome the horizontal stab position with their trim controls. (Caveat: current as of early 2019. When the final report comes out this may become a lot clearer, and may need revision).

There is an easy-to-understand video of the relationship between the jack screw and the horizontal stabilizer function here. Whenever the jack screw is moving the horizontal stab, a cockpit cue that this is occurring is the rotation of the manual trim wheels (seen at about 3:40 in the video).

Why is this activating during takeoff/departure?

That MCAS appears to activate(sometimes) during routine flight regimes is believed to be catching some crews by surprise - and not just the two accident crews. There is a recent article in Atlantic that excerpts some NASA reports by aircrews in the US about some cases of unexpected/odd pitch behavior in this model aircraft.

You can disable the stabaliser trim which will prevent MCAS from making inputs to the aircraft.

From what I have read, the other answers explain the cutout procedure correctly. It is also important to keep in mind Korvin's remark that trim anomalies can occur for a variety of reasons. What I have not seen mentioned often or clearly enough is a crucial difference that distinguishes MCAS from regular trim control, a difference which was apparently not communicated clearly enough to the pilots. As an article by Leeham news points out:

[MCAS is] not stopped by the Pilot pulling the Yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors.¹

It is easy to imagine that pilots who have hopefully practiced emergency situations, like trim runaway during ascent, are puzzled and lost when the ingrained procedures do not work. From the NASA incident reports published in the Atlantic it becomes clear that the FAA advisory after the LionAir crash was discussed among the pilots. Apparently there were a few incidents where performing the proper cutout procedure saved the airplanes, among them the preceding LionAir flight.

In other words, MCAS is a mechanism to correct an already present pilot error by hard overriding, so the pilot is not given the last word s/he has in most other situations, or at least only after a few button switches which are specially protected. It's also worth mentioning that even extreme yoke action, working on the elevators only, is not able to fully compensate extreme stabilizer positions, the control surfaces MCAS uses (apart from, I suppose, creating unpleasant flight characteristics).