With the dog in front position, and a reward placed directly behind the dog a few feet away, say your down command. If the dog downs, mark, and reward by telling the dog to "GET IT." The dog may leave the down to collect their reward
Gradually add distance. With your dog in a sit-to-front, ask them to wait while you step back. Give the down command, and mark and reward only if your dog downs with out coming forward. Reward by using "Get it." as you did in step one. Repeat this process until your dog can reliably down from the sit position at 15 paces away.
Add the recall. Repeat step 2, except vary rewarding the down with recall, vs, placement of reward to the dog in the down, AND/OR finishing the exercise with a recall to front.