Here's an excellent guide, which I had nothing to do with;
http://fallout.wikia.com/wiki/Reilly%27s_Rangers_%28quest%29Also, you can get this at the title screen by entering (`) (it's the key left of 1), then "addachievement 30." (thanks to LunaticPT for the correction)
If you have any questions, further advice for this game, or ways to improve this solution, please post a comment.