Keep it simple and learn from the masters. Christie, Doyle, etc...

Heres a Ted Talks video

Work backwards.

Never thought I’d ever be interested in watching a Ted Ed video until now lol

That might work, since I already know who the victim and the murderer are gonna be. And I already have an idea on how the first scene/chapter is gonna play out.

I just don’t know about any of the other chapters yet

Big tip that changed my writing, "Everyone has a secret. They all have somewhere they won't cooperate." Basically, if your murderer is the only dodgy character, the mystery is too easy to solve. So every suspect has to be hiding something. And often, when they do, try to hide it, they inadvertently help the murderer stay hidden. Even if your detective knows who did it, those other characters may be ruining evidence.

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Write an outline, filling it in and correcting it as your ideas develop and change.

Use a big white board or cork board with lots of connections, like a conspiracy theorist.

Write by hand whenever you can, pencil is better than ink.

When you get stuck with the actual writing, tweak the outline and play with the connections.

Listen to good actors read early to mid 20th century short stories and mystery novels.

No, I do not write fiction, but I do some writing.

That is classic Hercule Poirot.

These sound like some good ideas, I may try them. Thank you.

This honestly isn’t a bad idea, especially since I’m already planning to make one of my main protagonists at the top of the suspect list, despite being innocent. I could probably take things further from there.

REALLY?! I aim to watch one every day. I love learning

Not gonna lie, anything longer than a couple pages makes me grab the yarn and tacks. The tactile experience really helps get you in the mindset.

I guess it just depends on the person. If it’s about something I don’t really care about, then I likely won’t be interested, like when I have to watch one in class at school or something. If it’s something I’m passionate about, like this, then I’d be a lot more interested.

I get that. I used to be the same.

Over the years I discovered that the more I learn, the more I want to learn. If you ever need book recommendations let me know.

I read everything I can get my hands on.

Another thing, get out of your comfort zone. If youre not a fan of history (for example) I know for a fact I got a book that will change that.

History has always been one of my least favorite subjects, but the mystery novel I have in mind is based on a historical event (at least the backstory behind it is), so I will definitely need to do a lot of research on that, which I plan to do. And since it’s tied to my project in a way, I’ll probably be a lot more interested, and will enjoy it a lot more than if it was required like for a class or something.

Also, I saw the message you sent me. I will definitely consider checking out your sub, though I don’t know when or if I’ll have time to do so, especially since it’s pretty late right now where I live, and I have a big day tomorrow, so I probably need to go to bed here soon. Thanks for the offer, though!

Deborah Chester (who writes fiction and teaches it) has several books on writing genre fiction. The kindle editions cost about four or five dollars a piece and walk beginning authors through the process.

Plan out the entire story in advance, and make sure you spread hints (or implications) of the major reveals throughout the story, because the worst thing is if the reader feels like the ending or the twist is half-assed. You need minor reveals as well; you should be revealing impactful information like a slow drip--each individual piece of info could be a crumb about the world, the characters, or the events that transpired, but the important thing is to give small crumbs over time so that it doesn't feel jarring. You want a story that is well-contained, flows well from the beginning to the end, but also completely flips the perspective that the reader started with in the beginning by the time the reader gets to the end.

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The biggest tip I'd have would be to plan anything in advance, you can't go with the flow and see where a mystery is going, you need to know it from the start.

And stay 'simple' not in a way that the reader can understand what's happening from the start but they need to somehow get it. Don't go make the mystery unraveled by some letter arriving five pages before the end explaining everything (except if it arrives at the reader's door not in the book that would be amazing but not doable). Sparkles some evidences from the start but throw them on a wrong path from time to time, with a reason, they need to think it's leading somewhere (which it will, it will give them some more information they can use to try and understand who is the killer).

Don't had pages just to had pages, if you want to make the book bigger, had some personal story for the main character but don't send them on five clues that are just dead end, avoid dead end ad much as you can

For cosmic horror, never work out what exactly you are dealing with,

It doesn't matter who khasamanu is, we just have to put him back

For mystery, map out a "birds eye view" of the situation, then only tell it from one perspective

If your hero/detective is supposed to be legendary [like Sherlock or Hercule Poirot] then the ending has to be extremely gratifying. Each of the clues needs to fit into the punchline. A lot of writers do this by making clues to pieces of the mystery, for example, a messed up floor is at first something that seems like a clue to the main mystery, but instead is a clue to how somebody got to a window, which they then had access to frame another person. Something like that. You can also keep clues to other things, such as character development. An interesting thing you've likely seen a lot is a piece of knowledge that someone would only know if they were a doctor, or a soldier, or what-have-you. That's a really cool way to allow your hero to make a guess as to a murderer, without making it obvious for your readers. It also lets your readers from that world gratified, because "who could know this! I'm special and good at solving mysteries". And that's a feeling you want. It's like a hard crossword. If you can solve a really difficult crossword or you're watching jeopardy and get every question right, that feels way more gratifying than completing anything easy.

Other commentors are saying you should write the story after a full plan, which is fine, but that's not the only way to do things. You can create your points of interest in the story—you have a basic idea: A person [or more] dies, a killer [or killers] kills, and you have a horror element 2/3 way through, there's the setting for the first 15 minutes, person 7 gets a past event explained at this point, etc. Then you can mess around with the story until everyone is the murderer. Like in the other commentor's teded video, Agatha christie often switched who the killer was midway through. This allowed her to keep it fresh and also let her make sure that the novel is completely indecipherable until the very end. You're likely to give it away as you go just by adding the clues you're already adding. I don't think you're just going to create a random story and then magically solve it at the end. It's a lot harder to give it away if you don't even know who the killer is though.

If you go this route you might spend a lot of time rewriting and adding; meaning your book is going to be good.

If you like you can also add a neat twist, such as maybe your mystery knows who the killer is but not who the victim[s] are. People might like that, but if you aren't up to it you don't need it.

Make your characters captivating