The problem with converting a phrase like "take a leaf from your book" into (pseudo-)Latin is that the word "leaf" actually refers to a page in this context, which is a less common meaning than a leaf of a tree. Nobody these days refers to pages as leaves, except in this cliché. So it's easy for people to get confused and think that the cliché refers to tree leaves for some odd reason.

So when I first looked for a Latin equivalent to "leaf", I found "folium". That sounds good, and it even sounds like it's related to paper and books, via the current English word "folio". But it actually only refers to the leaves of trees, not the leaves of a book. Someone recommended I use either "scheda" (sheet of paper) or "charta" (sheet of papyrus) instead. In the end I went with "charta", since papyrus is just cooler than paper.

And declension, by the way, is the modification of the form of a noun depending on the grammatical case it takes in the sentence. And Athenae is of course a major city in Hellas, whose accusative form is Athenas, which I'm assured by a helpful Latin grammar Nazi is the correct case here.