Once you know your idea, it's time to develop the plot.
☐ Write Outline
There are two types of writers: plotters who outline everything, and "pantsers" who write spontaneously (as in, "by the seat of their pants"). Stephen King exemplifies the latter, though this distinction is widely debated. My view: you cannot have a complex set of plot continuity without building a plot. I recommend creating an outline as the simplest plotting method.
Scrivener is excellent for this — its cork board and table features make outlining intuitive.
☐ Perform Research
This is also where you perform any research required to back up concepts or statements you will make, or facts or technologies you will use in the story. Get the details right before you build too much around them.
☐ Developmental Edit
I regret not doing this on my first novel. Consider hiring a developmental editor to review your plot and suggest improvements to character arc and plot progression before you do extensive writing. It's much easier to rework a plot outline than a completed manuscript.