Try writing a page about each idiom. Do you take the idiom literally, or does it apply in some obscure way to a character, setting, or plot line that you already constructed? See the post directly below this one for an example of how to take an idiom literall or how to expand on the implied meaning of an idiom.
Idiom 1: Beating around the bush.
Idiom 2: Elvis has left the building.
Idiom 3: Water under the bridge