First of all, there are too many who do not actively teach this so thank you for your effort.
First, force them to listen. Ask them what they hear. "Do you sound the same as the person next to you?" "How does it sound different?" Once they know to expect this, they will actively listen more. Then start asking them if they are flat or sharp. they can play a note at the same time as another player or with a drone. If they are not sure, ask them to make their best guess and adjust for that. Then: "did it get worse or better?" Thats how they learn.
Take the time to talk about all of the things that affect intonation, and most importantly all the solutions: lip up/down (and what that actually means), airstream, fingerings, slides, etc.
More than anything else, be persistent.
Good luck