The most important thing to remember when you teach your dog to lie down is to make sure that you are not forcing your dog into the position. Putting your hands on the dog could cause them to become aggressive. Also, it encourages distrust and may lead to resistance in future training.
The easiest way to teach your dog to lie down on cue is to food-lure them from a sit position.
If your dog is not going into the "down" position through the traditional lure, you may need to become a little creative.
You can lure the dog under your knee to get them into the "down" position. Sit on the floor with your knee bent to make a low bridge, and move the very desirable food under your leg, so that the dog has to crawl under your leg to get the treat. Once their elbows and rear end touch the ground, praise heavily and offer the reward.
Also, you can capture the dog’s behavior when they lie down in a natural setting. If you see that your dog is lying down on their own, offer praise and treats to show that this is a desirable position.