I have tried probably eight thousand different plotting methods over the last ten or twelve years, and I now stick with something like three or four? A lot of it depends on what the story will be, but I will still use at least two different plotting and structuring methods before I start writing the book.
A whiteboard is my go-to if I don’t feel like I have enough story to fill out a beat sheet or plot diagram yet. I keep a huge-ass whiteboard in my room and have it divided into eight sections, four down one side and four down the other. The left side is for the “main storyline”, and the right side is for “B plots” (and/or C, D, E, etc. plots.) The four sections on each side are labelled Act 1, Act 2A, Act 2B, and Act C. I keep it as available to me as possible, and when an idea pops into my head, I write it on a sticky note and add it to my board in the appropriate location. (Or a rough approximation of the appropriate location.) This is a great physical brainstorming process for me, and I find being able to see all my ideas constantly keeps them moving around in my brain longer, which in turn helps me connect the dots more thoroughly. When I feel like my board is full of plenty of ideas, I transfer it into my preferred plotting chart.
Save the Cat! Writes a Novel is my go-to for plot structuring. If I feel like I know a lot of the basic plot beats already, I’ll jump right into organizing using the STC beat sheet. I highly recommend reading the book if you’re not familiar with this idea, and especially if you really struggle with saggy middles and the organization of your story beats. This is also a really great breakdown of the structure.
A synopsis is my third (and usually, final) main plotting device. I spend a couple hours writing everything out in as much detail as possible, and do my very best to connect dots where I notice something being skipped over. I’ll often leave notes for myself to do research on things, fact check, brainstorm, or re-plot something. I also have a tendency to do this by hand. Writing by hand helps me think through things more thoroughly as I write them, and I have less of a chance of skipping over vital details. Later I may transcribe the handwritten synopsis into Scrivener so that I can easily refer to it.