Cross your t's and dot your i's

Pay careful attention to details and complete all the small, finishing touches of a task. It means being thorough and meticulous in your work.

This phrase comes from the literal act of handwriting, where the letters 't' and 'i' require separate strokes to add the horizontal line (cross) and the dot respectively. The expression emerged in the late 1800s when penmanship was crucial for business and personal correspondence.