do one's homework
Be well prepared, as in Steve had done his homework before the meeting and could answer all of the client's questions. This usage transfers a school assignment to a broader context. [c. 1930]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.