Return to Sender - A Long Walk
I'm serious about it being a long walk. You'll need to go all across the Mojave Wasteland for this one.
You can start the quest by going to Camp Forlorn Hope. Sergeant Reyes should be in the command tent 24/7. Talk to her at her radio post and she should mention the recent problems that they're having with the radio reports being intercepted. Agree to take the quest to get the new encrypted radio codes.
With the codes in hand, travel to the six different ranger stations. This part of the quest is simple, once you make it to the actual ranger station you just have to talk to the comm officer and give them the new codes.
Ranger Station Alpha - This one is on the path to the New Vegas and fairly close to the trading post and Boulder City. You should be able to just follow the marker.
Ranger Station Bravo - Bravo is far to the northeast and is fairly close to the Bitter Springs base. There's some mountains in the area, but the path to the ranger station should be fairly straightforward and clear. Watch out for fire geckos though.
Ranger Station Charlie - As covered in the link, it's possible, and in fact quite likely, that Ranger Station Charlie will have been destroyed by this point in the game. Don't worry, they accounted for this. After you check in at the other five stations, the quest will update normally and you'll just report the loss to Sergeant Reyes.
Ranger Station Delta - This should be an easy walk, since you can probably get straight to it from Forlorn Hope.
Ranger Station Echo - This one's to the south and close to Camp Searchlight. Be careful of any Legion soldiers and ghouls on the way there, and try to stick to the west of it. The area's a little rocky and the best shot seems to be coming in from the left side.
Ranger Station Foxtrot - This is probably the hardest to reach, since it's off by itself to the far northwest. It's actually really close to Jacobstown, so if you can swing by on your way up the mountain. I had trouble getting to it from the south. The best way seems to be going up the path until you're about even with it. You can then veer off and basically just "climb" down the slope right into the camp. It worked for me at least and seemed to be the path of least resistance.
Once you've been to each ranger station, report the success back to Sergeant Reyes. Things can't ever go as planned though, so we've got a little more work to do.