Donna Tartt drafts her novels by hand, in spiral notebooks, with pens and colored pencils. The messier the pages get, the closer she is to a finished draft. The physical process breaks down like this.

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She writes in longhand, in patterned notebooks, then types the pages up once they're too tangled to read.
She's described buying notebooks with 'silly patterns and colors' as a memory system, in an interview with Omnivoracious.

Longhand First, Always

Before anything reaches a keyboard, Tartt writes it out by hand. She scratches things out, underlines in different colors, and cuts the paper up with scissors so she can move sentences and paragraphs around. When the handwritten pages get too messy to follow, she types them into the computer and prints fresh copies.

The mess is the method. Rearranging by hand lets her feel the shape of a scene before it hardens on a screen.

The Notebook System

Tartt buys notebooks in the fall, when school supplies hit the shelves, and stocks up on ones with distinctive covers. The covers do real work. Each pattern is a memory cue, so she can recall which notebook holds which scene.

In the fall when the school supplies are in the store, I'll tend to buy lots of them so I can get just the kind I want: silly patterns and colors are, for me, an important aide memoire, a mental filing system.

Donna Tartt, to Omnivoracious

While finishing The Goldfinch, she's said, several of her notebooks had Beatles album covers, which made it easy to think 'I wrote that in the Hard Day's Night notebook.'

Color-Coded Drafts

Once she's typing, Tartt prints each draft on a different color of paper. For The Goldfinch she tracked a pink draft, a grey draft, and a blue draft, so she always knew which stack was newest. She's credited the color-coding trick to an old French teacher. Read the full Donna Tartt writing routine for how this fits the wider process.

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