Put your dog in a SIT (see my other guides for help with SIT). It helps DOWN if you don't have to use the word SIT so your dog doesn't think DOWN is always prefaced by SIT. Try just your hand signal.
Don't give a treat for sit yet. Use the same treat and slowly lower your hand to the ground. Your dog should be interested enough to follow. The hand signal is a flat hand facing down.
Once the dog is relaxed with elbows and butt on the ground, give him the treat. Sometimes to keep the dog interested, you have to place the treat between your middle and ring fingers and let them lick
He should look natural and relaxed in his DOWN. Never force your dog into a DOWN. Sometimes you just have to wait them out until thy get tired and plop down. Don't forget to release them!!
Don't forget to check out my other dog training guides!!