Teaching your dog to lie down on command isn't hard.
Have your dog start off in a sitting position. Put a small treat in your hand and let them sniff the treat. Don't let your dog eat the treat though or get ahold of it. Slowly, lower the treat to the floor. This should entice your dog to gradually lower themself to the floor to follow the treat. As your dog is lowering themself, say "lie Down". Eventually, your dog will be lying on the floor, praise your dog in a positive tone than give them the treat.
Only give your dog the treat while they are lying down. Do not give it to them if they are standing back up. You need them to know they are doing good. Dogs are like humans, they love to be praised. eventually, your dog will lie down whenever you say their name and the command "lie down"
Hope this helped. Let me know if any questions. Happy Dog Training to You