With idioms, students already know the words that make up the expression. But since idioms aren’t to be taken literally, they still need to learn the meaning. Instead of teaching idioms like you would other vocabulary terms, why not build off what they already know?
There’s a lot of vocabulary to learn, but thankfully plenty of words are related to each other. Learning words by associating them with each other helps us to remember those words later. You can help students establish and strengthen those connections in your students’ minds.