一个一个 / 一步一步【yí gè yí gè / yí bù yí bù】

一 + measure word, said twice, means "one by one" or "one at a time". It goes before the verb, usually followed by 地.

The measure word is the one the noun normally takes: 本 for books, 件 for clothes, 张 for photos.

Step by step, little by little

With 步, 天 or 点, the same frame describes gradual change: 一步一步 "step by step", 一天一天 "day by day", 一点一点 "bit by bit".

Where it goes wrong

English puts "one by one" at the end. In Chinese it comes before the verb.

• × 老师检查我们的作业一个一个 → 老师一个一个地检查我们的作业

个 isn't the default here. Use the noun's own measure word.

• × 他一个一个地看照片 → 他一张一张地看照片