A little bit harder then sit. There are 2 methods:
Method 1
First, get your dog to sit. Hold the treat out in front of the dog, just out of reach. With your other hand, gently push your dogs front legs from behind. Keep trying if they resist. If it really isn't working, try method 2. After you do that, they should lie down. Praise A LOT. A good scratch and GOOD BOY/ GIRL will do. Don't forgot to give them the treat. ;)
Method 2:
Start by getting your dog to sit. Hold the treat out at your dogs nose and slowly pull it away from your dog to the ground. They will try to go after it, thus lying down. If they try to get up, hold their rear. Praise in the same way as method 1.