(1) If you haven't Googled "How to Handle a Malicious Narcissist," that's the least you should do. Consulting a few professionals would be better.
(2) Never say "yes," when the Republican candidate asks in that sneering tone, "Well, can I respond?" JUST SAY "NO!"
(3) Understand that you are not moderating a debate. You are making it possible for Madame Clinton to get a word in edgewise before the Republican candidate growls, "You'll go to jail!" or "She needs a drug test!" or "String 'er up!"
(4) It is your bounden duty to put an abrupt and definite halt to such remarks: "Stop that, Donnie! We don't talk like that, Donnie! Behave yourself!" would--yes, really--be appropriate. If you feel a need for decorum, and I think you should not, then say, "Mr. Trump! That language is inappropriate! You have two minutes to re-phrase."
(5) Before the debate even begins, lecture the candidates on expected behaviors, making clear that any and all inappropriate remarks or threats will be censured. Each candidate is expected to speak to the issues.
(6) Understand that you really don't need to know the issues yourself. You don't need to bone up on the latest ghastly Donaldism or Wikileaks. This election has never been about "the facts." It's always been about personalities.
(7) Consider the mistakes of moderators in the last two debates. Flummoxed by the Republican candidate's bad behavior, they ignored it, behaving as though his menacing motions and words were something to be ignored. They aren't. Step into the fray and call him on his shit.
(8) When you don't do this, Hillary Clinton is left in the position of the mom who's supposed to mop up after the huge mess, with no Dad backing her up. That's really not fair. Moderators so far have allowed her to be bullied. You then create a situation in which Trump is allowed to be predator and she is forced to be prey.
(9) When a candidate refuses to play by the rules of polite society, then you cannot stick to these rules yourself. You must be active.
(10) If you remain passive, and if the Republican candidate wins, then there will be a catastrophe followed by a post-war period and a need for a de-Trumpification process. Even if Madame Clinton wins--and please, let's hope she does--the media needs to plan, with good government, a de-Trumpification process that will neutralize the hatred stirred up by Trump and which he will otherwise work to keep alive.