Best way? To value your sanity above everything. Above possessions, above your marriage, above the opinions of the mutual friends he will recruit to press his case.
Be aware he will not let you go easily. In his eyes, you exist only to serve his whims. It is not his whim that you leave and have an independent happy life that does not involve him. He will use multiple strategies to get you back under the thumb. Forewarned is forearmed against these strategies.
Flying monkeys - what the wicked witch sent to do her bidding in 'the Wizard of Oz'. These will take the form of friends, family, colleagues, near-strangers. He will lie to people, feign being distraught, wail about 'if only we could meet and talk she'd see we are good together!'. And these flying monkeys will feel sorry for him, be manipulated by him (he's a master manipulator, remember?) and set upon you, pressing his case for him, make you feel confused again (because it is gaslighting by proxy) and put you under the pressure of multiple voices telling you he really loves you and leaving him is a mistake. And surely if you're the only person feeling this way, they might be right? That's how it will feel. But no, of course they're not right. They don't know what went on behind closed doors - he saw to that. So you stand firm, tell them they are wrong and that if they cannot respect your wishes then to leave you alone. Or to fuck off, depends how angry you can get with them. Anger is good, by the way. It's easier to stay strong when you're angry.
Beware the 'sunk costs' fallacy - do not be sucked back in based on 'if you quit now all those years together are meaningless' blah blah blah. What's done is done and cannot be undone. You've heard the phrase don't throw good money after bad? Well replace 'money' with 'years of your life'.
Be prepared to walk away from finances and possessions. You are worth more than the whole shebang. He will probably try to suck you back in many ways - to collect your personal stuff, to talk about selling your mutual home, to talk about settling bank accounts, etc. Try and keep it at arms length. Take all your important stuff with you when you leave, get a trusted friend to collect anything later. (Warn them he will try to turn them into a flying monkey.) Big stuff like house sale - via a solicitor. Do not engage, it's an opportunity for him to browbeat you.
Do not forget that who he is now is the real him. No nostalgia for the early days before his mask slipped. That person is gone - indeed, they never were in the first place, they were an illusion spun by a narcissist.
So, on a practical note - do you have somewhere to live? Access to money? Emotional support?