1
general::
Phrase(s): to the letter
exactly as instructed; exactly as written. • I didn’t make an error. I followed your instruction to the letter. • We didn’t prepare the recipe to the letter, but the cake still turned out very well.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Precisely, as in If you follow the directions to the letter, you can't go wrong. Letter here refers to the exact terms of some statement. [c. 1800]
American Heritage Idioms