There isn't a good way
Unspoken expectations are incredibly pernicious. When it comes to humans, engaging them in a way counter to their unspoken expectations, about a non-standard non-normal activity (like roleplaying) is always going to be difficult. Doing so while also playing a game that makes use of creative storytelling skills (high mental energy) is going to increase the difficulty considerably.
Successfully doing so will almost certainly depend on the individuals involved. Skills to modify expectation are soft skills, and hard to describe. Different individuals will have slightly different expectations and different means will work differently on each individual.
That doesn't mean it's impossible. However it does mean that setting down how to do so with some kind of step-by-step instructions will almost certainly be incorrect, very easy to apply wrongly, or ultimately unhelpful.
Instead, I'm going to say that this is a social communication problem, based around expectation, with some minefields in the form of those expectations already covering for/providing a framework to avoid embarrassment and dissonance. Approaching it in those terms, as a slightly awkward social situation like being stuck with your sister's fiancee who you don't like on a camping trip, may help solve it better than thinking of it as a 'roleplaying problem'.
However specific to roleplaying games, there are a few minor tricks that may help a bit (I'm dubious they help more than a 'bit' however);
Give your game system a name and frame the actions you're asking for them to perform as 'part of the system'. 'Do you want to play Steve the Innkeeper' may lead to mind boggling in shock, but '
- Okay as part of SNAVES, anyone can Bid to get the option to play as
- 'how does that work'
- 'well I give you three Lines on the character and then you-'
Assigning names to things you just use as innate narrative mechanics and vaguely pretending it's all part of some kind of actual system (even if you freely state it's something you thought up or even that it's not actually written down anywhere) will fit part of the existing 'rpg' profile - there's rules, you're following them (apparently).
Simplify initial scenario. Dumb it down. Normally you run political intrigue with 5-8 moving parts and people know what to do via shorthand like 'Jim you want to usurize the pivot sidewonkwards yeah?'. Don't do that. Cut variables, simplify the in-game scenario heavily. Simplify the out of game scenario - ask people to do less narrative actions, and heavily explain what they are doing even if they already know. You're explaining it from step 0 to step 33 in way too much detail for the benefit of the new guy. Take more of the MC duties on yourself and simplify things to give the newer player a better chance to learn and also to acclimatize.
Use dice rolls more. Dice are fun and easy to understand - high roll good, low roll bad. Involve them in things you'd usually decide by people dictating the narrative outcomes of the scene, or on things you'd normally not bother to roll for (the trained soldier shooting a target 20m away in good lighting from a braced firing position).
Get help. You've identified this problem and put it into words. Get your table to do the same thing, and crowdsource the solution. Doing this stuff yourself can work but if the whole table is trying to ease people into your different meta it's probably going to work better. Keep in mind you might run into the problem in reverse - people not understanding how people don't/can't grok their current mode of play, because that's what they are used to.
Godmode. Like anyone interacting with something they don't hugely understand, the new player will be more predisposed to fail. In D&D that means their character fails rolls or dies or whatever. At your table I'd imagine it's more that the group doesn't take up their idea and it falls by the wayside, or outcomes they don't prefer happen more often. Take that, shake it round, and turn it upside down. Have them win more. If necessary, have them win in a D&D sense (their character kills the enemies) and in the more narrative sense of your table (the party runs with their idea for the scene). Like bribing a child with sweets, players like winning. In the short term it will keep their interest, which might otherwise be turned off by the strangeness of your game compared to their previous experience. This will give time to potentially come to appreciate the more narrative tone.
More clearly delineate the things you have control over and what the party/table has control over. It's likely not clearly delineated, possibly intentionally so - if necessary, make some fake delineations instead. It is a lot smaller of a mental leap from 'the GM controls the enemies' to 'the GM controls these specific factions' than to 'the GM controls whatever he feels like controlling, and so do the players'.
Be willing to go through a lot of candidates. Being creative in real time with other people is hard. Narrative focused games require more of that skill than D&D. Add a cultural disconnect and a lot of people are going to nope out at the first hurdle. A significantly effective means to solve this is to simply be willing to just keep going through people until you find one that sticks, and not take anyone leaving/not showing up anymore personally. This is to a large degree true in roleplaying in any case - often you want to be quite selective about who you game with to make sure your investment of energy is repaid with interesting ideas and a good time.
Finally, you should keep in mind that what you're referring to - the role of the GM in your game - is the tip of the iceberg here. It's the obvious thing they point to, but the disconnect is subtler and broader and that is what is fueling the issues they are having. If it was just that the GM does not do things other GMs do, it would be less of an issue and I doubt you would bring it up as a major problem. However having seen exactly this disconnect quite a few times (both at my own tables and others) I can confirm that it makes games harder, causes players to drop out (despite enjoying the game by all reports), and is in general a cause of un-fun. I've noticed an uptick in this kind of problem recently, which I attribute to.
[..Rules-heavy DnD5e-as-combat-simulator is currently the major source of roleplaying game players and the culture of altgames that permeated the release of earlier editions of D&D has not made the jump that D&D has to mainstream culture. Most players now have not played Traveller and Conan and GURPS a few times to give them the idea that TTRPGs can proceed in a different fashion to Keep on the Borderlands and Dungeon of the Mad Mage. ]
In some cases I can refer to players who did not understand the concept of homemade adventures rather than store-bought. They had to have it explained several times and were edgy about the prospect, thought that it would lead to imbalance and bad times etc.
That is the depth of the difference of view that can be available.
To stop waffling, to solve your GM-role-view problem you'll likely need to at least interact with the broader disconnect of soft-rules narrative game vs hard-rules combat simulator game. That follows the usual rules of doing so which are complicated and hard to describe. Show don't tell, simplify, remember you know it and they don't, provide incentives, etc. Retreading that ground would go on for pages and pages, and there's reams of advice on how to teach and how to connect with people that aren't really RPG specific available in the world.
TL;DR - your problem is narrative vs combat simulator game style/experience rather than specifically GM-role. You'll probably need to put effort into making the game more accessible/understandable to people outside context to help combat this. How you do that is variable and the difficulty varies depending on the type of person.