dot the i's and cross the t's
dot the i's and cross the t's {v. phr.}
To be careful, thorough, and pay close attention to detail.
"The best way to get an A on the final exam," the teacher said, "is for every one to dot the i's and cross the t's."
Compare: MIND ONE'S P'S AND Q'S.
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Source: A Dictionary of American Idioms