Walk them through a previous design project. Pull out all the design artifacts, from concept design to detailed specifications. Show them video from any usability tests that took place. Demonstrate the process that was followed to go from a high-level problem statement and concept(s) to a detailed and final solution. And don't try and do this all at once… take your time to tell the story about the design and the steps taken.
Then, throw them in the deep end – give them a small project to design, and leverage the same methodology you just detailed. Mentor, encourage, and be constructive when being critical. Let them learn by doing.