HI bml1105,

Since there are so many idioms that could show up on the GMAT, you shouldn't try to memorize them all at once. Certain idioms show up more often than others and certain groups of idioms share common traits. I generally recommend that Test Takers learn idioms in small "chunks." It's also really beneficial to see how an idiom is used in a complete sentence, so you can see the context of how the idiom is used. I highly recommend reading the explanation (or watching the video) for every sentence correction that you do (even the ones that you get correct) - there might be an idiom or style rule that you didn't even realize was there.

GMAT assassins aren't born, they're made,
Rich