Yep... I'm plotting again! Book #9 is with Fiona (my editor) and so I'm starting to sort out the plot for book #10.
I tend to plot in layers... I get my key scenes worked out, then layer over the top of them the scenes that are mid-point between them. And yes, I do all this on index cards. Sometimes these are virtual index cards (in Scrivener), but a lot of the time they are actual index cards.
Both the key scenes and the mid-point scenes tend to go on larger cards (6" x 4"). The scenes that come in between go on smaller (5" x 3") cards, and at that point, my dining room table becomes unavailable as I lay them all out and make sure the plot flows smoothly with no massive gaps. No major issue at the moment, as no one is allowed in anyone else's house!
First up will be the key scenes. None of these are especially unusual - look at any "How to plot a Novel" piece and they will be there... The hook; the inciting incident; the first plot point etc. They help me lay the foundations of the book.
Getting the points in between is also enormously useful - what other key things need to happen between the major points? Does the protagonist need to have found something out by then? Does another character need to have appeared by then? Sometimes these can be less of a scene plan (at this stage) and more of a list of things that need to be covered, but that's okay. I can work with that.
Book #10 has two stories that weave a dance together, with both of them resolving at the end, so I know I need to have a tight idea on my plot, otherwise it will end up like spaghetti. How 'pantsers' write is beyond me. I absolutely have to have a plan before I can dive in.
Once I have The Plan, I don't actually have to write the book in order, which helps enormously if a scene is sticky and just not flowing - I can leave it alone, write another and come back when my brain has sorted it all out.
How do others sort out their plot? If you're a 'pantser', how do you not end up with spaghetti? I think I would just end up with an unholy mess if I didn't plan.