To join Raiders faction, you need at least 120 reputation points in Slaver's Guild. You can get the points for example by doing Bring Little Jesus Mordino's briefcase to Metzger quest or by becoming a slaver and selling slaves.
- Metzger will send you to a bar called "The Hole", left of Metzger's building.
- Find a guy named Stokes there. You must have reputation with the raiders no worse than -190, to talk to him. After a long conversation, he will send you to the Raiders' base (a new location on the map, near NCR). Go there.
The farmer's quest
You will find Jim Raynes in the base. Ask him about joining the Raiders and he will tell you to get rid of a farmer, Cooper, living in peace with his family (In another location : The farm). Go to that farm and kill that man, however, you do not need to hurt anybody else to complete that quest. Killing the child will make you a childkiller.
- After reporting it, you will be able to see another Raider base - near Broken Hills.
After joining revist Stokes. He will have some stuff for you. Amount of stuff might depend on your charisma.