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I Put a Spell on You - Introduction
As mentioned above, this is just for the Legion side of the quest. If you want to do the NCR side, look at the other guid for "I Put a Spell on You." If you want to do the Legion side of this, you'll need to first finish "The Finger of Suspicion" for Vulpes Inculta at the Fort. Once you complete it, you'll learn that the Legion has a deep cover agent inside Camp McCarran. You'll also be sent in to see if he needs any help with his latest big project.
If you don't want to take that route, or you can't take it, then you may be able to do it by building up a reputation with Caesar's Legion without making the NCR hate you. You can then get the NCR side of the mission by talking to Hsu and talk to Curtis. He should reveal himself and ask for help if you're well liked by the Legion.
On the flip side, if the NCR considers you a terrorist, this may be incompletable without some serious luck and good use of disguises. Curtis still counts as an NCR officer until you talk to him, so he, and other NCR officers, will probably attack you and make this very difficult.
Regardless, if you can still do the quest, go to Curtis and introduce yourself. He'll ask you to help out by also getting the quest from Hsu so that you can control it from both sides and help Curtis stay hidden. Do so, and then get ready for the big sabotage mission..
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I Put a Spell on You - Framing Crenshaw
In order to frame Crenshaw, first look in the garbage can outside of the ticketing desk on the first floor. Grab all of the bomb making ingredients inside and also grab the blueprints for the monorail bomb. Take this evidence up to the Concourse. There should be a marked footlocker by one of the beds in the barracks area. I only saw one soldier patrolling, so it should be easy to just crouch down and slip all of the materials into the footlock while he's not locking.
You may have already found Crenshaw on his little out of the way patrol. He's right around the back of the base by Contreras' Supply Shack. You should be able to cut through their and wind up right by Crenshaw. If you want, you should be able to just kill him normally. He is all alone back here. You can also reverse pickpocket a grenade onto him, or even use C4.
Note that there is a special option. If you talk to him, you can ask him about his favorite pranks. You can then mention that you like pranks too and tell him that "you just pulled the pin out of someone's grenade", if you have 40 points in Sneak. Run back quickly and he'll blow up just as if you planted a live grenade on him. No one should notice.
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I Put a Spell on You - Sabotaging the Monorail
There are two guards at the door who have orders to shoot on sight if anyone besides trusted NCR soldiers approach the door.
If you are trusted by the NCR, you should actually be able to walk right past them. The same should be true if you have some NCR armor to wear as a disguise.
Otherwise, you can catch them at a changing of the guard. I believe that this should happen at 9 AM and 9 PM. I can confirm the 9 AM time. Just wait outside and watch. As soon as the hour turns over the two guards should walk away. Wait for them to walk a fair distance down the hallway and then slip behind them. Listen for any warnings though, since they won't officially quit their guard duties until they're at the end of the hallway.
Go through the door to the monorail, and then jump onto the train. Go to the back of the cabin and use the vent to plant Curtis' bomb. Run off the train and enjoy the fireworks. You can then just walk back to Hsu and report Crenshaw's guilt. Just tell him about the "evidence" that you found to convince him. You'll actually get about 100 caps for your successful investigation.
Talk to Curtis to confirm that you're done and receive your reward from him too. This should wrap up the quest.