Tips for creating murder mysteries in my games?

Whatever you do, don't mix up who the killer is behind the scenes just because the player guessed it correctly before you wanted them to.

@Atlas48 First off, know from the outset whether you want to run a genuine mystery scenario, with an actual truth under the hood where the point is to overcome the challenge of finding that truth, or engage in mystery-*shaped* storytelling where the goal is to end up with a tale that resembles a mystery from the outside while not actually taxing the players' brains. Advice varies wildly depending on which you're doing.

Don't put important details behind failable skill checks and just dead end it there.

Like if they find a book with ciphered text, you might be tempted to be like "make an intelligence + investigator check to decipher it", and if they fail be like "you can't figure it out".

It's better to do some sort of degree of success or succeed at a cost so the game keeps moving forward.

Like, on a bad roll they translate it but whoops awaken an angry spirit that's now attacking them. Or they make some progress, but realize they need the key to fully crack it. The note in the margin says it's at such-and-such flophouse, owned by the PC's most annoying rival group.

I've done too many "you rolled .. 0? Ok. Well you have no idea what this altar means" and then later regretted it because the players didn't have a vital clue.

In my eyes rolls basically need to be Yes Ands or Yes Buts.

Roll well: Yes And you figure out some extra information/figure it out quickly so you have advantage on your escape/you look impressive in front of the guards who did let you into the room

Roll poorly: Yes But you take a bit too long figuring it out, now guards are walking into the room/one of the guards escorting you into the room points something out to you making you look a unobservant/you accidentally break something and you've now left evidence you were here.
For my skill checks I set multiple DCs for a roll including automatic information. So depending on how high they roll the more information they get but they always get something. This is especially true for information gathering spells. Things like getting based doors or guards they can fail. But these kind of failures just drive them to other options for getting based the barrier such as breaking down the door or getting the guard drunk.
- I've found that when the players hit an outright failure, a lot of the time they just draw blanks or zero in on this one specific solution. It's a weird tunnel vision.
  
  Like, they want to talk past the doorman and he says no after they roll. Good players on their game will then think about other options. Sneak in the back. Set off an alarm. Impersonate someone who lives there. But i've just had so many players that just get stuck on this, and will try to spend 10 minutes on "What if I ask him nicely?"
  
  I've started including a spiel about this in my session 0. "If an obstacle in the world has exactly one purpose in the story, and you attack it dead on, you may fail. Especially if it's not also your strong suit. For example, there is a doorman of a fancy apartment building. His entire role in life is to look at people, and only let them in if they're authorized. If you walk up to him, not authorized, and go 'Hey bro let me in', that will be a very hard check. That is shooting fire at the fire elemental. Disguising yourself will be easier, but still is in his domain of 'Looking at people and only letting authorized folks in'. But going in a back door so he doesn't see, setting off the fire alarm so he evacuates, calling on the phone and telling him his car has been towed, those ideas hit him where he's weaker."

Lay out what happen before end. Personally I start from the murder and go back to find the why.

After use The three clues rule

Hit your players over the head with multiple clues, and make sure that it's hard to get dead-ended.

The following is a ttrpg classic that I periodically reread: https://thealexandrian.net/wordpress/1118/roleplaying-games/three-clue-rule

Seconding this, use self-contained scenes and clues that lead you from one scene to the next without requiring a strict plot to guide you.
Ya I think he's said something about replacing that advice with node-based adventure design or something, but this article by itself has helped me improve tons of mystery scenarios by itself that I think the advice works as is.
- It's not that he replaced it, it's that he built on it. The Reverse Three Clue Rule used in his node-based design articles ("if the players have at least three clues, they'll draw at least one conclusion") is a corollary, not a refutation of his previous advice.
  
  The main way it's changed since he wrote this article (and since he wrote his Node-Based Design series, for that matter) is that he distinguishes between clues and leads, which he didn't at the time.
- Good point. Node based design works particularly well for mysteries.
  
  I think the general suggestion for having lots of redundant clues is still relevant, regardless of how the GM plans the adventure.

When doing that, I follow what I saw done in LARP.

Pre-gen character, making sure that everyone has a secret, a couple of goals, and relationship with character. Then I find an "in-game" reason to keep everyone in the same-place (snow storm, hiding from the police, magical myst, on a boat), Finally, I had a couple of clues which may reveal some secrets over the game-location, and sometimes a couple of "events", but usually letting "brew" is enough to have the plot revealing and the betrayal occuring. As any game with some PvP component, keep some time for debrief/cool-down, it can be emotionally intense

As opposite to LARP, I would still have a bunch of NPC, who would also have their goal/secrets. However, I would try to keep the "big secrets" and "culprit" within the PC.

IMO it's harder to incorporate this kind of things in an exiting campaign, or you go to a more classic investigation (which are nice too),

There's a YouTube channel called Pointy Hat that has a really great video on murder mysteries, or mysteries in general and how to theme them and drop clues. Highly recommend

Be careful with red herrings and other false clues and leads. Most parties will make their own false leads and other conclusions without any prompting based on something completely random or unnecessary. So limit this as much as possible and make it obvious when investigating why someone is innocent if they are. The investigation is part of the fun.

Also try to make sure there are multiple ways to get an important piece of information with the general recommendation to be three different ways. These should involve the players special skills / spells /etc. to encourage and not penalize people who doing what they do best. These channels should be flexible as well because the party may do something you didn't expect such as interviewing the mice in the castle to see what they heard that night. So if they do anything they should get some information even if its a duplicate of something they already found out.

When I was doing my most recent adventure there was a ballroom scene where the party was trying to find out something suspicious. There was two mysteries going on: a secret fey run raucous boozed filled Mirror Ball and some extradimensional horrors had body snatched people to eat their brains. So I had some set DCs for various secrets they could learn from automatic to DC 35. So when they did something that might get some information I picked one and they learned it. Some of these secrets were given multiple times from multiple sources. They ended up getting all of them except who is the extradimensional horrors by befriending and raising the social standing of the busty redhead wallflower who was secretly the queen of gossip by some massive performance and persuasions checks. So be flexible about how they end up learning the secrets because they can and should all be given as information to the party even if it makes the killer obvious.