To acquire this quest, speak with Crowley inside the Underworld. He will ask you to kill a few people with an incentive for shooting them in the head, then collect keys from them. You can kill them any way you'd like if you just want to finish the quest, or you can actually just acquire a key from each of them. You can pickpocket the key or use speech challenges to get them in a variety of ways.
Now there is another option to finish the quest instead which may be better for you. Once you have the three keys from Ted, Dukov and Dave, head to Fort Constantine and use the keys to open some doors and retrieve the power armor held within. This is still considered a successful completion.
One final option is to speak with Tenpenny and warn him that Crowley is alive and wants him dead. He will offer you money to instead kill Crowley. Doing so will also complete the quest.
See You Gotta Shot 'Em in the Head for more specific info.