Dot one’s I’s and cross one’s t’s
This idiom means to be very careful and thorough and to pay close attention to details. When we forget to dot our i’s or cross our t’s it is indicative of careless work.
“There’s an inspection coming up and all your note books will be examined, so don’t forget to dot y our i’s and cross your t’s.”
When the teacher told her students this, she meant it both literally and metaphorically.