Wear a nice suit and some bags over your shoes so no fibers will be left behind. Use a rifle purchased legally but shells illegally so those can't be traced (and you can't get busted for illegal possession of a gun), fine a nice high place, and snipe people. Don't tell anyone, period. When you're done, take off the clothes you are wearing while killing, put them in the plastic bags you wore around your shoes, and donate them to a homeless shelter. Break the gun, take it back and demand a refund or something. A replacement. Any extra ammo, sell it or give it back to whoever you bought it from. Donediddlyscrumptuos.