One by one; one each; handling things in single units or sequentially.
箱の中のお菓子を一つずつ配ってください。
Please hand out the sweets in the box one by one.
質問には一つずつ丁寧に答えます。
I will answer the questions carefully one at a time.
書類を一つずつ確認して提出した。
I checked the documents one by one before submitting them.