Explaining what collocations (otherwise known as "combinaisons de mots" in French) are can turn out to be rather tricky sometimes. So instead of using many words, give your students a few good examples of collocations and the way we use them in speech. THESE TWO EXERCISES provide some useful examples as well as push your students to think a little bit more on the subject. After they finished doing the exercises, you might probably need to say a few words on the importance of learning collocations, on their extreme usefulness not only for fluency but also for accuracy etc. But it will be much easier for them to understand the concept when they already have sth to look at.