What you're looking for is a non-traditional outline.
Now a regular outline is very linear and has headings and subheadings and may either:
- Bring on terrifying flashbacks of high school English.
- Just be too linear for what you need in this particular moment.
But an outline doesn't have to be so formal. A storyboard is a type of outline. So what may work for you is the storyboard format but with text, not pictures.
My spouse writes actual comic books (he's not an artist by any stretch of the imagination) so he's laying things out into panels so the artist knows what to draw where. But he also has to keep track of multiple storylines.
He's a huge fan of index cards. This way he can take the elements of each story thread and move them around as needed.
Index cards don't work for me (I tried). But I do have a wall covered with character information (keeping track of my characters is much harder than my plot, not because it's harder in general but because I have so many characters).
If you're trying to map out plot, index cards or full/half sheets of paper might be worthwhile. There are plenty of computer versions of this but sometimes you have to manipulate things physically to get it all to work. So I'd recommend using paper to start, then transferring it on to a computer if you prefer.
Don't draw a thing (if you feel an urge to, then do it anyway). Just use character names, locations, and the main plot beats.
Do this on the dining room table, floor, wall, or wherever works for you. A roll of painter's tape is a must-buy here, especially for the wall, but even elsewhere.
- Write or print out the big stuff. ("Marta and Lester have first kiss" or "Earthquake!" or "Sally starts kindergarten," etc.)
- Lay it out in a line or grid or even some other configuration if things are happening simultaneously.
- Move things around to get the order right.
- Add blank pieces of paper where there are gaps.
- Fill in gaps with handwritten cards, or printed as things get established.
- Go through the events and add characters. Who is where when?
- Go through character actions and add locations and events. Where are they and what is also happening?
Once you have at least a good first draft layout, photograph it (do not neglect this part! Even if you live alone with no pets and never open the windows). Repeat periodically.
You don't have to have it perfect to start writing. If you have the first third of the story mapped out to your satisfaction, put out some chapters. You may find the plot shifts as you begin to write anyway. And that's okay.
If there's a particular chapter calling to you, write it. The first chapter I wrote for my novel is about 3/5th of the way into the book. Because it wouldn't leave me alone! I've written two more out of order.
Your outline is your tool, not your master. Make it easy to change but also easy to follow.