1) Find some retarded chicks.
The best place is a mall food court. You'll usually see a bunch of mentally disabled people trying to sweep and mop and return used trays. They're usually involved in some program with their group home that tries to make them feel "useful in society" or some crap. If your local mall is upscale enough to employ wetbacks, then try Nathan's Funtastic Fun World on a schoolday.
Also, finding a retard that is out in the world ensures that they are not TOO retarded. Yes, there is such a thing. Though the chick drooling in her wheelchair while trying to bite her ear may ensure you don't get a dry blowjob, she probably won't give you too much hip action.
2)Isolate her from Authority Figures.
Retarded people are conditioned to obey the authority figures in their lives (such as a family member or a counseler at their program) and trust them in all things. It is important that you break those bonds of trust, as they will prevent you from getting some double-chromosome poon.
Be sickeningly nice to your retarded lover. Inform her that the people in charge of her are really mean and that you would be much nicer to them. Give them tons of candy and soda pop. Tell her that the person who takes care of her is trying to poison their food and shit like that. It is essential that they trust you above all else. Encourage them to run away if you have to. If they need to, you'll let them sleep in your closet for a few days.
3)Getting them in the sack.
Retarded chicks either have no conception of shame about their sexuality. In that sense, they would be the perfect woman if their eyes weren't too close together. In fact, you will have to kind of shut yourself off, go to a happy place, while doing your retarded chick. She will likely not be in very attractive physical condition. Her muffled sounds will be, well, retarded. You will usually have to do all the work, but hey, it's the same if you use Rophynol. They have typically not have had sex before, so the first time will hurt them and feel weird, so you may need to stuff a sock in their mouth the first few times so people don't think your strangling Snuffleuffagus in your apartment.
4)Kick em to the curb.
This is harder than it sounds, seeing as you have usurped the authority figure spot in their lives. Lock them out of your house, and you will just find them sitting on your porch days later, muching on pop tarts and not take the hint.
My best suggestion: drive them out someplace unfamiliar, like to a different city, tell them your going to use the bathroom, and ditch them someplace. They aren't dogs, they aren't gonna be able to find you across half the state. Just lose her and forget it.