The word focus has many meanings. Concentration is the definition that most people think of first. However as a writer focus can mean many things. It can encompass discipline, habits, and anything else that will aid you to become a more aware and mindful writer.
All writers need to exercise focus but for the crime writer the focus must be maintained to ensure that you have placed your clues correctly, you are being completely fair to the reader, your characters have revealed or hidden something from the reader, and you have worked out the balance of your scenes. Every scene must drive towards the conclusion, which is the revelation of the killer.
So my first advice regarding focus would be for you to write notes about the scene you intend to write. Jot down the first sentence of your scene, leave a few lines and write the last sentence of your scene. By doing this you know your start and end points and clarified what you are trying to achieve. On the lines in between those two things, write the key points that you must cover. These will include actions by your characters, a seeded clue or a little revelation that might be a red herring taking your reader up the garden path or a huge clue that is so obvious the reader will be immediately suspicious that it might be a red herring. Then when you begin to write, you know where you are going, but I would always advocate leaving a bit of leeway so that if your brain suddenly thinks wait a minute, if I make Roger do X, that will confuse everybody. By making notes, you will immediately aid your focus for the scene.
What can affect your focus? Knowing what you are going to concentrate on in your writing session, dealing with your surroundings, whether that be noise or interruptions or making sure that there is nothing in your immediate environment that is going to distract you. This includes family! Do not be apologetic about your writing. This is important to you and nobody, be it your mother who thinks it is a little hobby she can interrupt for a chat about nothing, or email, phone calls should be permitted to distract you and take your time away.
Focus is a time-limited activity. Some writers use “timed sprints”. These can be 30 minutes of timed concentration where you just write, followed by physical activity, be that walking around the house for 10 minutes jogging up and down the stairs or, as in my case, putting on a short 10-minute playlist and dancing. The physical act of moving about re-energises the neurons in your brain and makes you more effective when you sit down for your next timed sprint.
Where you write. When you research the habits of successful authors, you will find that this is not a one size fits all situation. The crime writer, Ian Rankin, for example, can only write in his office at home in Edinburgh or his farmhouse in France. Nora Roberts used to say her office was a notebook and wherever she and the notebook went, that was where she wrote because she could still maintain supervision of her children but jot down her story at the same time. Some lucky writers, of whom I am one, have a dedicated room in their house that is an office. Others write on the bus going to work or have to squeeze in a couple of hours when they come home from work.
Some writers go outside the home. JK Rowling wrote her first Harry Potter book in the coffee shop. I have used and libraries. A new place to write occasionally kickstarts your brain into a new form of creativity. If you do decide to occasionally or even all the time, write in public spaces, you need to have a portable office that you can take with you. In other words plenty of pens, your laptop, (make sure it is fully charged!), notepads, earplugs and music, your notes and anything else that you think you will need.
I use an A5 “activity” diary. This gives me a Monday to Friday A5 page-a-day book in which to write down my objectives for each day and how much time I will devote to each activity. At the end of each week I can review the past week, write down what went well, write down what didn't and what will make me happy to achieve in the next seven days.
Reviewing your week in this way gives you the impetus to carry on. It is also invaluable in assessing where you are in a project and if you need to change some of your forward planning. In short, especially if you are also holding down a job, and/or have a family as well, you need something that is going to keep yourself on track.
Being a slave to your habits will stultify your brain, so change them about from time to time. Get those brain neurons firing in a different pattern. Old habits are hard to break. They fight for their existence and they are very persuasive, because if you are leading a very busy life, it is so easy to give in to the habit. But just because you can do something does not always mean you should.
Focus, discipline, habit, call it what you like. But the plain fact is that without it not only will you never write a book but you certainly will not be able to write a crime novel. I hope some of the ideas I have given you above will help you in your writing journey.