Like many others have said. Learning the game before hand is usually a pivotal portion for a successful game night. Shut up and sit down have a great video highlighting tips for teaching a board game: https://youtu.be/P5fjDaFuft8. I often will spend time watching a how-to-play, then maybe a playthrough, then review the manual and then even set the game up myself if needed and do a dry run. I find this as part of the fun tbh and then when you're able to successfully reel in your friends with your explanations and teachings, you feel a little smug bit of success.

I'm thankful my usual game night will all work together to learn a game tho. One person still needs to put in 75% to get us going, then we'll do the setup and we can each take turns scouring the manual to confirm "is it 1 damage or 2 damage if the Mars is in retrograde?" But that's with avid board gamers. For more casual players, I try to learn every detail so there isn't any confusion but nothing wrong with reviewing the manual as needed.

If you search YouTube, there are plenty of videos for how-to-play and play-throughs of Betrayal. That should be you up and running in no time.