I think they'll probably be very hard to understand for your students. I suppose the easiest way of getting them to remember what they mean is to tell them where the idiom comes from.
For example the idiom "Beyond the pale". It means to have gone beyond what is acceptable, so someone may have stolen something from someone for example.
If you take it literally it makes no sense, because you can't really go beyond a lack of colour. In this instance the word pale derrives from the Latin word "palus", which is a stake. These could be forced into the ground to form a fence. If someone were to go beyond this fence then they had left civilisation.
I'd plan ahead if I were you, which idioms you plan to teach, and attempt to research where they came from so that you can explain it to them.