You've written books. That's not the problem. The difficulty you're having is that a MYSTERY has to be done partly with the left brain, the logical/spreadsheet side, while a regular book can come mostly from the right side, with prodding from the Girls in the Basement and the Muses you are channeling.
So now you're going to try a mystery. And this means you'll have to sit down early and do a little outlining. Ouch! Let's just call it "plotting." That's what our perp has been doing, after all. We'll assume you have been living with your sleuth and his/her milieu for some time now, and you've got a reason for him/her to be involved in solving this crime.
You've decided on your crime/victim (and method: hitting, strangling, etc.) and perp and at least two other people who will be suspects. But your READER can't figure this out until your sleuth does!
I always sit down and figure out three ways that the crime COULD have plausibly been committed, or hidden, or whatever. Then my sleuth has three theories she can hit upon, and the third of them can be "pretty close." I never ever give her the EXACT crime, because then what would Our Villain have to spill at the end where she confronts her/him and gets the full story? (grin)
The intrepid mystery writer now needs to make up real and false clues that will point to the villain AND ALSO point to the other suspects to keep the reader guessing. Don't be afraid to make these off-the-wall; when you're actually writing the scenes, you can always change them or tone them down.
Once you have a list of clues and red herrings (or fake clues), you will need a timeline for the book, showing the order in which the clues will appear. ALL the major clues pointing to the real perp (motive, means, opportunity) MUST be planted in the first third of the book. (I am NOT making this up.) This is part of the "fair play" sort of mystery so that your reader has a chance to figure out whodunit. Clues must be planted in such a way that they're hidden or their meaning isn't known till the end of the book.
So how in the world will you keep readers from seeing those clues and flinging the book against the wall? You'll need to use subterfuge and sneakiness. Fortunately, these are major traits of most writers.
Our first trick is to HIDE THEM IN PLAIN SIGHT. Remember Poe's purloined letter. They have to be sitting there on the mantel, but seem part of the collection of Star Trek toys that is arrayed around them. If you're going to skewer the victim by having shishkabob speared through with (deadly) oleander branches, have him wander through someone's water-feature-filled back yard or go to a picnic--and notice the flowering oleander all around. It's right there in plain sight.
Another trick is to hide the clue in an inventory, litany, or other pile of ordinary items. Let's say that a scratched Swiss army knife is a major clue. Then when the sleuth goes through someone's pockets--all right, you don't want people to hate the sleuth for snooping, so make it that she is leaving an office party and goes into the bedroom where everyone's coats are piled on a bed and grabs up her purse and her windbreaker/raincoat, only it ISN'T hers, and she does notice that it's getting a little tight and thinks that's because she gorged on those little dates filled with cream cheese and crackers with Brie and jam . . . but when she gets home and shoves her hand into that pocket to get her keys, or whatever, she finds that they are not there BECAUSE it's actually someone else's coat. Nothing's in the pocket but a pocket knife and some stale sticks of gum. She has to get into her house, so she digs out the hidden key in the garden or calls someone--the plot drives us forward. And the coat and knife are forgotten until you want her to remember them.
I've been dinged in the past for having a character take inventory of a purse, knapsack, or other container, but it is a great boon to a writer. You can show character by what is being carried. You can show what is important in the story by what's found. And you can hide something the character will need later, such as a flashlight or bus token or diary! But I digress.
When you can't hide a clue, just show it with flair RIGHT BEFORE some BIG THING happens. IMMEDIATELY grab the reader's attention and focus it on something big that seems really important at the time. For example, your hero overhears the perp's phone conversation and hears a big clue. He is shocked, but before he has time to think overmuch about it and fit it into the puzzle, a siren goes off outside or a deafening crash is heard upstairs. The old masters loved to use a SCREEEEEAM or the dousing of the house lights. Our Intrepid Sleuth dashes upstairs or outside to take care of the crisis--neighborhood fire, broken mirror upstairs, second dead body discovered by young screamer--and by the time this is dealt with, both your hero and your reader have forgotten the overheard conversation UNTIL near the end, when he is putting two and two together. Maybe even four and four. Aha! He (and your reader) remember now.
As far as putting some numbers to it: a cozy series mystery is expected to be 85K words, or at least between 80K and 90K. This is 360 pages. As I mentioned before, ALL the clues have been put in by the 30,000th word--the end of the first one-third of the text--which should fall on or about the 120th manuscript page. The last two-thirds of the text is spent tracking down clues, following rabbit trails, suspecting the wrong people, etc. The last five chapters MUST ramp up the tension. Either your sleuth has guessed the perp (and is wrong, and is about to be confronted with the truth), or is very close. If your chapters are about 15 pages, then you have 10-11 chapters full of clues and red herrings, and chapter 25 is your final wrap-up.
I am a pantser and not a plotter. This means I don't rely overmuch on an outline per se (that's PER SE, Latin, not "per say," BTW, if you're following along in the home game), but I do keep a list of scenes sometimes or even just a list of details. I need to know the general shape of the story in brief. Sometimes I will even have a file or synopsis that is expanded, including what I think each chapter will probably hold. ("We need to have her learn to sight-read before she discovers the Mystical Sheet Music and plays it to open the hidden bookcase in chapter 4!")
This can all change during the writing, of course, but I always know the ending of my books before I start writing. Otherwise, to me, it's like starting off on a trip without a map and wandering around hoping you finally get to somewhere interesting that will make it all worthwhile. You might get there, but it can sure take a lot longer. You might instead run out of gas before you get anywhere. Or end up on the South Side of town without a paddle . . . or even a baseball bat for self-defense.
Even though the plot is the main thing to many mystery readers (or at least to the reviewers, especially the ones on Amazon), remember that a mystery is not "what happened" ("the jewelry store was smash-robbed"), but WHAT HAPPENED TO A PERSON OR PEOPLE ("Joe lost his shirt when those thugs smash-robbed his store and the insurance dragged its feet about paying up fair and square").
Readers MUST care about your detective and somewhat about your victim or his/her mourners or what he/she leaves unfinished. Sometimes readers should even care about the suspects and perps. WHY does she want or need to solve the mystery? (Usually, she or her sister is a suspect. Or something like that.) What is the connection she has to the victim or to his/her relatives or organization? She has to care about what happened if you want your reader to care. Will it matter that much if she comes up with the REAL PERP? It'd better.
You may think that a bad guy is a bad guy, but don't forget that most people think they are doing the reasonable thing, that they are in the right or are at least justified in their actions and beliefs. Most "baddies" do not see themselves as baddies. They are just on the other side of what YOU think of as the right and proper action. They see themselves as doing God's work, getting revenge, putting things right, or whatever--not just "getting away with something," usually, although that can play in as a factor. So when you make up the motivations and reasoning for your perp, be sure it is reasonable from HIS or HER point of view, however stinky that POV may be for you and your sleuth. People have reasons for what they do. People ALWAYS have a reason for their actions, even if you think it is a bad or false reason.
Most villians (unfortunately) look like the guy next door, or are handsome/beautiful, or whatever. They aren't a Snidely Whiplash in black leather and a twirling waxed mustache. They look like normal people: a relative, your neighbor, someone who works at the same job or goes to the same school as your heroine, is on the hero's sports team (maybe even as the coach or sponsor), etc. She is on the PTA committee with your victim. He used to date your victim or her sister. What I'm getting it is that the real perp should be connected to your victim in one of these "normal" people-trees. Don't just make him an anonymous stranger who blows into town and commits a random crime. PLEASE don't do another serial killer sociopath with multiple personalities!
Don't forget the victim(s), even though they may die in the opening. A victim can be sympathetic or detestable. If everyone hated him, it is easier to create a big list of suspects. On the other hand, if nobody seemed to hate him, you have a tougher mystery for the reader to solve.
If the victim is your old boss . . . that can work. It worked in that crazy movie I caught on cable the other night, HORRIBLE BOSSES. Even though that was a pretty screwed-up black comedy. It also worked in my novel NICE WORK, available now on Amazon and in all bus station restrooms.
This should give you some idea how different it is to write a traditional mystery. Maybe you ought to stick to romance . . . or maybe you'll try your hand at this. After all, if *I* do it, how hard could it be? LOL!
(I know this was "TL;DR," but the right type of person read through to the end. I commend you. Now go write a mystery. Send me the link when you're in print!)