For this achievement, you have to kill 20 zombies that are lying down.
Throughout the campaign, you will see zombies that are lying completely flat on the ground. To tell if a zombie is dead or not, simply aim your reticle over the body. If the reticle turns red, the zombie is alive. I was able to complete this easily on Episode 1, but these zombies are common and you will have many chances over the course of your playthrough. On easy, this should be an simple achievement. Just walk up slowly to the zombie on the ground, and shoot it in the head.
I am unaware if knocked over zombies count for this achievement, I completed this by slowly approaching and shooting zombies in the head that were lying down.