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Back in Your Own Backyard - Introduction
You can start up this quest by talking to Ranger Milo at his blockade. He's guarding the road between Novac and Nelson. If you talk to him for a bit, he should mention the three dying soldiers on crosses in the center of Nelson.
At this poitn he will give you a choice to help him. You can either agree to go out and do a quick mercy kill on the soldiers, or you can go in and save them.
Note that the quest will bug out if you pick one and do the other, so make a choice and stick with it.
For a rough rundown, the mercy kill is pretty easy. You should actually be able to find a good cliff overlooking the camp, which will let you quietly snipe the three NCR soldiers. One shot should kill them. You can also sneak in and do it at close range when the patrols aren't looking, if the Legion doesn't hate you yet.
The rescue mission will earn you more reputation with the NCR than the mercy kill, and more importantly, if you have Boone then it will count as a marker for the companion quest. Doing the mercy kill seems to count less for earning Boone's trust than saving the men. Of course rescuing them will require an actual assault on the camp. If you plan to do this, then I suggest you do it alongside "Restoring Hope." Note that if you already finished the quest and saved Nelson, the NCR doesn't save the crucified men. You can still do it even after the NCR controls the town.
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Back in Your Own Backyard - Saving the NCR Troopers
For the mercy kill option, you can walk to the left side of Nelson and get onto a cliff or just stand on a good hilltop. Crouch down and make sure that you're counted as hidden. You should be able to fire three quick shots and retreat with having to deal with any of the Legion soldiers in the town.
If you want to actually save them, then just charge right into town. You should probably try to do this at night. Engage the exterior patrols, but don't try to clean out the camp completely. If you kill Dead Sea, then that tends to have a nasty habit of bugging out Restoring Hope, which is pretty important for securing the region for the NCR.
The three crucified men are in the center of the town on the large crosses. If you follow the road you should just have to take out the men at the checkpoint and a few patrols inside town. Go in quickly and untie the three troopers, then fall back and only engage guards that are chasing after you. As noted, you can also just save the three hostages during the raid on Nelson in Restoring Hope, which seems to be the safest way to take care of both quests.