If you write mysteries, you need to have a victim, a killer, and some red herring suspects. You also need witnesses. When I started writing cozy mysteries, I had a hard time until I came up with the idea of using a mind map. I put down the salient facts of the crime. I wrote down the important details. Each sheet has the name of the victim. It also has the cause and time of death, the location where the death occurred, and what the murder weapon was. Then I described the crime. That became the victim sheet.
Then I decided who the killer would be. I wrote down the killer’s name, motive and opportunity. Then I asked the killer for an alibi. That is, I wrote the killer’s alibi down as though the killer was telling the investigator. I also included the clue that points to the killer. Then I went further in to detail on what actually happened.
I then did some of that for all the suspects. I noted each suspect’s name, means, opportunity, alibi and the clue that points to them. I wrote the alibis out as though the suspect was talking to an investigator. I would write something like, at the time of the murder I was riding my bike on the beach in full view of twenty people. I nearly ran over little Jimmy Carstairs because he ran away from his mother into the path of my bike. That sort of thing. It’s the same thing for the witnesses. I would write down the witness’ name and what they saw in their voice.
Of course, for these sheets to work, you have to have already determined who the victim, killer and so forth are. You should also complete character charts for them as well. It will make it easier to write the alibis/confessions from the character’s point of view.
I also include the times that the witnesses saw whatever it was they saw. It helps if you know that three days before the crime, John saw the victim and someone arguing. Or another character saw the victim do something to annoy someone. Then you can add it to the time line of the story which helps in plotting. You can also see where you need to put the various clues.
I’m not saying this is what all mystery writers do, it’s my process. It works for me. It may or may not work for you. This is a suggestion intended to help you with your own writing process. If you find it works for you, great. If not, you should come out of it with a better understanding of what you need to do in writing a mystery. If nothing else, you got some writing practice in.