Reinforce with treats when your dog lies down naturally before adding in a verbal cue.  

Hold a treat by your dog's nose then move it down towards the floor guiding them into a down. Reward once they lie down.

Say "down!" each time you lure down then reward. Repeat until lying down becomes habitual when they hear the cue.   

Slowly incorporate giving the verbal cue without showing the treat to transition off the lure.

Only give the reward if your dog completes the full down action after hearing the cue.

Over time, step away from your dog before cueing down to proof the behavior without proximity.

Reward your dog for lying down on cue during walks and play sessions.