How do people construct a plot for a work of long fiction? Everyone has different methods. Some just write and have an instinct for constructing a fine storyline. Others use post-it notes, index cards, charts and diagrams to help them.
Me - I have done different things at different times, depending on what I was writing. With BACKLASH, the book more or less wrote itself. By the time I reached BURNOUT the complexity of having the story interwoven with two other novels meant that I had to develop a big wall chart to help me.
Right now I am working on a treatment for a movie. (This is a project I was invited to work on, so I will have to hold back from writing about it too specifically for the time being. At such time as it goes public, I'll say more.)
Back to the question of plotting - how do you plot a movie? This is far easier than a novel because there is far less plot. You can more or less hold an entire movie plot in your head at one time. A novel is more that a head-full.
But I still find myself using a specific method to help me. In this case - index cards. I've already worked up a short treatment. To develop that into a full treatment, I simply take each scene that I know I will definitely need and write a couple of lines about it onto a blank card. I can then lay the cards out on the floor and figure out which cards definitely have to come before or after which. The process will inevitably help me to understand extra scenes that are needed - and where they have to go.
The end product is a stack of 40 or 50 cards, each with a mini scene description. That's the plot. Simple as that. Wish me luck.