I have basically written what follows here about half a dozen times already in replies to questions in various Kings of War Facebook posts over the last few years, so I figured it might be a good idea to finally post something more substantial here, so that either I can refer back to it myself, or point people towards this for more info when similar questions pop up in the future. I am not a Kings of War authority, but I did run a number of these smaller "learner" games in 2nd (some of them detailed in Battle Reports here!), so hopefully I learned some things and can impart some wisdom here to folks trying to bring this wonderful game to others.
So, you are here because you to run some introductory games! Huzzah! My biggest single piece of advice is to not treat these as normal games. It doesn't matter if you play a flawless game here; it doesn't matter if you win. The two goals here are to teach the game and to have fun, as this is still a game. Everything you do should be in support of these goals.
LIST PREPARATION
If you are building all the lists, then remember that the meta is yours to dictate! You don't need to consider including anti-flyer tech or add in spellcasters if you don't want to run them. If Flying or Nimble or Shambling units make your head hurt, minimize their appearance in the lists or leave them at home if you need to. I would suggest that you build these learner lists casually and don't be afraid to run "bad" units! Evaluating units is part of the game after all and discussing the lackluster performance of a particular unit with your opponent can lead to a good conversation and engagement. (Was the unit just used incorrectly? Was this a bad match-up for the unit? Would it work better as a larger or smaller unit size? These kinds of discussions can get folks more interested in the game.)
I was using Easy Army for all of my list documentation, and so my lists were always legal for unlocks, but I gave a preference for running Regiments, as they were more likely to stick around in combat for a turn or two. Usually each list had some Troops slots also, to showcase unit sizes and to open discussions on chaff units and the like. While legal to run, I quickly came to avoid all hordes (even for Ogres) and to avoid most magic items in my learner lists. With the former, hordes were just too powerful and led to grindy games which were not only not much fun, but they also didn't showcase the game well. The latter really only got exceptions for the Inspiring Talisman on an Undead Necromancer or Abyssal Efreet, as those were such staples in those lists/collections at the time that I wanted them in my learner games too. If your collection dictates that a list still needs a magic item, write it down on a card, and let your opponent decide where it goes, placing the card with the unit. (If you want, you can even curate a few options, to let them pick both the item and then the receiving unit.) Having the physical card means one of you is more likely to actually remember the item in-game, and even if the choice of unit seems obvious, having a newbie actively make that decision does help them start to understand list building more.
Often at least one unit was usually repeated (two regiments of Shield Wall or something), as this repetition helped players understand things quicker, and built in a little redundancy to the lists. Monsters were used sparingly, as like hordes, many were either too expensive or too powerful to effectively run in a balanced learner game, though monsters around 100-125 points should be fine. Each list usually only got a single inspiring hero, so the focus could be on the units.
I ran my games at 750 points, and would probably continue at that level in 3rd Edition. Games at this size gave me around 4-7 units and a hero per army, and the games are quick. An hour and a half was often about enough time for two games, but an hour per game is still doable if there is a lot of discussion going on. Discussion is good, so don't be afraid to pursue it in lieu of a second game. Lastly for lists, remember to bring list printouts to the game!
TABLE PREPARATION
If you can, set up the board and terrain beforehand as this saves time. Setting up symmetrically (so negating a side advantage) can also help speed things up. With 750 points, space is good, but you do not need a full table. I've run games these size on 4' x 4' and even 3' by 3' tables. If you are running this on a smaller-than-usual size, reduce the depth of the deployment zones and preserve the 2' buffer in the midfield whenever possible. I've found that preserving that buffer maintains a more usual "pace" to the game, so no one is taken aback by very early charges by fliers or cavalry.
Terrain rules can get confusing, so I would recommend picking a few (maybe 3 max) types or terrain and repeating them on the table, and running more of the same terrain types (multiple mountains, multiple fences, etc) rather than a wide variety of types. Terrain really livens things up for wargames, but learning terrain rules alongside of a brand new game can be overwhelming for new players. Having say, some impassable houses, some fence obstacles and some forests scattered about is enough to make the battlefield believable. If you are playing at home or using your own terrain, don't be afraid to use terrain sparingly in these games. Terrain is also a collection that takes time to build up no matter the wargame. Do what you can and try to keep things simple.
Lastly, I try to have a set of colored dice and measuring device for each player now. I have tried using just one set before, passing them back and forth to emphasize the I-go-U-go rules Kings of War has, and really mark whose turn it was, but it was really awkward in practice.
GAME SET UP
For learner games, I take some unusual moves. If you focus on just one section of this post, focus on this. I skip the deployment phase entirely. It saves a ton of time and removes all the potential "feels bad" moments than could arise with you forcing these game-defining decisions on someone who has never played before. I definitely appreciate the strategy and the mind games that can happen during this phase, but this has no place in an learner game for me.
I will pick a very simple list for myself, often Kingdoms of Men. Since those units are historical, they are easier for opponents to intuit. ("Archers eh? I should probably find a way to charge those and shut them down." or "Knights eh? I should probably find a way to not get charged by those... hey, how do spears work in this game anyways?") Running a simple list for me also reduces the number of complex decisions I need to make in a game, meaning I can spend more time discussing and teaching instead of pondering my next moves.
Once my opponent arrives, we decide on sides, and get down to it, I show them my list and plop down my army, pointing out my units and showing the different stats. My deployment doesn't really matter, but I often go with a "traditional" approach: cavalry on the wings for a pincer; back and turtled to protect a war machine; or a refused flank to try and roll up a board edge. Overall, just plop your army down, and don't do much thinking. Any "goofy" deployments on your part can be fodder for discussions with your opponent. Once you have set up and explained your list, then your opponent gets to deploy. This order of setting things up lets them immediately start putting their new game knowledge to use, placing their units down to try and counter your plans if they want to.
Lastly, I'd often cede first turn to my opponent. You'll need to walk them through the turn, but having them go first keeps them engaged and also avoids potential "feels bad" moments if your list has some shooting components. Setting up and immediately getting blasted is a bummer. So let them deploy and then keep the ball rolling, letting them go first, and working through that first turn together.
RUNNING THE GAME
I'd recommend "Kill" for a learner game, as it is easy for new players to grasp. The game has a lot of fighting-specific stats and abilities, and ball-parking combat math is important in all scenarios, so pending some time to reinforce a newbies combat knowledge is generally a good thing.
Even on a rerack, I'd suggest replaying "Kill". While I do very much like the scenarios Kings of War offers, I like to use any rerack to just introduce the deployment phase, and save scenarios for future games. Keeping the objective the same in the rerack also helps reinforce everything they've learned that day, since they're still striving for the same goal.
HANDLING MULTIPLE PLAYERS
I like lists at 750 points as the 1v1 games were very quick, and the few times that extra players showed up it was easy to incorporate them without the games dragging or becoming too messy.
With four players, each grabbing a 750 list and running a 2v2 can be a very nice way to get some comradery and discussions going. Four different armies are on display, and a variety of interaction is possible so different players will see different things.
What about 3 players though?
If all you have is two armies, I would advise that you step aside, and let them fight it out while you teach them both the game. This goes back to the goal of these games being to teach the game and have fun. Sure, that hour may be a little less fun for you, but you still get to share this with them.
If you have a multitude of army lists at 750, I would still suggest you step back. Running two different armies is going to drain anyone. A newcomer won't effectively be able to do this, and you're already teaching the game. Taking on teaching and running twice as much as anyone else at the table in inadvisable.
If you can boost one army list up from 750 to 1500 (you planned ahead bringing the same army with a different list, or are playing at home), you can obviously bump one side up to 1500, and run two smaller lists against it. A 2v1 like this is definitely possible. In this situation then, it seems like you, as the experienced player, should take the larger army. As above, I would advise against this. Again, in these games, you are likely the only one to have read the rulebook at all. You are already trying to teach the game, and you do not need to add in this extra mental work running a larger army to things. Offer up the larger list to one of the new players. This not only eases the mental efforts on you, it also gets both new players engaged for the entire duration of the intro game.
So, that's the gist of my experiences and general advice. Simplify things wherever you can, and remember that the two goals for these games is to teach the basics, and of course, have fun. Good luck!
Edit/Addition:
I missed this. Mantic has a free version of the 3rd Edition Ruleset available for download from their website. If you want to try the game, it is hard to beat "free" for an entry fee.