If you've never done it before, congratulations, try to keep it that way. If its your first time you're going to want a tube of greese paint or some chalk. Draw an x using it from the left eye to the right ear, and the right ear to the left eye. The convergence point is the thinnest part of the skull. You're going to want to use a small caliber hand gun for this next part. .22 is what most vets recommend. A police service pistol is way too much and will make a huge mess. You're going to shoot the horse at the center of the x with the gun held very close, but not directly on the horses head. The idea is you cannot miss from this distance, but you also won't create a pressure problem that causes the gun to misfire. The idea with all of this is its the least likely for mistakes. In an emergency, such as shipping a horse overseas on a plane, a trained attendant will use a higher caliber handgun, such as a .38, and skip the x, instead eyeballing it.