Using a food lure is the easiest and best way to teach a dog a new cue. Keep in mind that teaching a dog the down position using a food lure can sometimes take longer than you might expect. This is especially true of very small and very large dogs. Remember to have patience and keep at it. Once you can successfully food lured your dog 3-5 times in a row into the position, you may add the cue word, "down."
Below are some more pictures of the class practicing the food lure with Sit, Down, and Stand.
Finally, remember that ANY time your dog performs any behavior you are looking for, to give the dog LOTS of good praise. "Good dog!!!" with a very excited, happy, up-beat voice. I like to say, pretend its the best thing that's ever happened to you. The better you are at marking your dog's proper behavior, the easier your dog will learn to do what you want. Below is a picture of me demonstrating my excited, "good job" to a dog who's done well.