dot the/one's i's and cross the/one's t's

1. 一丝不苟; 重视细节

You'll have to make sure that you dot the i's and cross the t's while doing the work.

做这工作时, 你应该做到一丝不苟。

on the dot

1. 准时地

She was told to arrive at 8 o'clock on the dot.

她被告知八点钟准时到。