〜とおり(に)・〜どおり(に)
exactly as, in the way that
Verb plain form / Noun + の + とおり(に) · Noun + どおり(に)
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とおり means "just as, exactly as": 先生が言ったとおりに、やってみた, "I did it exactly as the teacher said". 通り is "road", so the image is following the same path.
It follows a plain verb (often 言う, 思う, 書く, 見る) or a noun with の: 説明書のとおりに, "according to the instructions".
Directly after a noun, it becomes どおり: 予定どおり, "as planned"; 時間どおり, "on time".
思ったとおり, "just as I thought", is a common set phrase. Compare ように, "as, like": とおり is stricter, "exactly as", which makes it the natural choice for instructions and plans.
Examples
先生が言ったとおりに、やってみた。
I did it exactly as the teacher said.
説明書のとおりに組み立ててください。
Please put it together exactly as the instructions say.
思ったとおり、彼は来なかった。
Just as I thought, he didn't come.
予定どおり、会議は三時に始まった。
The meeting started at three, as planned.
私が言うとおりに書いてください。
Please write it down exactly as I say it.