Adichie is famously reluctant to over-explain her process, and she says she doesn't really plan. What she keeps returning to is character, clarity, and a standard she calls radical honesty.

The Direct Answer
Character-first, with little planning and a demand for radical honesty on the page.
She's said good fiction needs a kind of radical honesty that no class can teach, per an interview in The Muse.

Character Drives the Story

For Half of a Yellow Sun, Adichie did heavy historical research on the Nigerian Civil War, then watched her early drafts go wrong because the events were steering the book. Her fix was to put the people back in charge. She has said the characters have to drive the narrative, with the history sitting behind them rather than in front. She also resists rigid planning, and has admitted she sometimes has to invent answers about her process, because she doesn't map a book out in advance. The book reveals itself as she writes it.

Radical Honesty on the Page

The standard Adichie holds herself to is emotional truth. She calls it radical honesty, and she's clear that it's the part of writing a workshop cannot hand you. A writer can take every class going and still not reach it. For her, this is the real work of fiction: telling the truth of a character even when it's uncomfortable, and refusing the easy, flattering version of a scene. She's talked about the willingness to give too much of yourself away, to go wherever the story takes you.

There is a certain kind of radical honesty that is required to write true fiction, and a young writer can attend many classes and still not have that.

Chimamanda Ngozi Adichie, The Muse

Trusted Readers and Letting Go

Adichie leans on a small circle of draft readers, people who wish her well and who tell her the truth. She's said both qualities matter, and that a good reader can be more useful to her than another writer. When a passage fights her, she doesn't force it. She's described leaving difficult work alone for a while, then returning to it once the pressure eases, which keeps the writing from turning into a joyless grind.

For the wider picture of how she works, see the full Chimamanda Ngozi Adichie writing routine.

What You Can Steal