First, let me start by saying that every writer is different. This is not the sole way of how to begin writing a novel. I simply find that this process works best for me.
Usually when I get an idea for a novel it is essentially focussed around one conflict or an image of a character. I try to make the characters honest and real by giving them basic attributes that normal people have. (It is not necessarily imperative to these written down in the story. On the contrary there are times when the reader prefers to get to know the character through there words and thoughts alone. However, for the writer, it’s good to know their physical attributes, the sound of their voice, what they think is funny, what gets under their skin and their personality etc.) Personality is key in a novel because even if the character grows in the story it needs to be believable.
Believable is not the same thing as real. Even if the book is fantasy or sic-fi the reader will still want to try to place themselves in the story so once you give a character core morals, make sure you stick to them or have a damn good reason of why they change.
Next I like to start with an outline. If there is a conflict or a needed growth for a character what scenes are needed in the story to get there? For example, perhaps you are writing a crime novel and you need to create scenes that build up tension leading to the case being solved. Or, maybe your writing a story about two people falling in love. What needs to happen in the story for feelings to bloom?
Next, ask yourself ideally how many chapters do you want? (this is also subjected to change, but it’s helpful to try to fit scenes in an outline so you have a foundation for each chapter.)
Start with outlining the first chapter…where is your character? Are they talking to someone? Are they doing something? How will this pull the reader in and help set the stage for the rest of the story? The first sentence is usually the hardest. I like to think of it as if it is a hook for a paper, ending in a thesis of where your story is headed. After outlining the first chapter start labeling chapters, for example 1-18 and put scenes you previously imagined in the linear order where you think they would fall naturally in the story. I find it helpful to have an idea of what the ending will be before I start writing so that I can create scenes that will lead up to it. You may find that the ending doesn’t fit as you get half way through the story and you can change it, but initially having one helps you put the puzzle pieces into perspective. Writing a novel is a grueling process, but I find that this helps me create my first draft fairly quickly.
Anyways, I hope this was helpful! I am currently working on my third novel and I find that this process never fails me.
P.S. If you are writing a novel, even if you don’t find this advice helpful, I still congratulate you on the hardy task of creating a written world!