Place your dog in a sit.  Gently take the dog and slowly push it over onto her side as you repeat the command, "DEAD DOG."  Once she is on her side with her head down, give her a treat.  If you have problems getting the dog on its side, you can also do one stage of sit and one of down; then turn her on her side.

If teaching the BANG command, only give it once as your dog begins to go on its side.